Well, after way too long, I am finally writing this!
Maybe that's for the better, as now I have used both pens constantly on a daily basis.
PRESENTING: The Old Dog (Pelikan 400), The New Dog (Pelikan m400) and the inks: Pelikan 4001 Blue, Diamine Ancient Copper.
INTRODUCTION: The Pelikan 400 was a Old Dog before it came out in 1950--with a celluloid binde in tortoise colors, a squarer design, and a classic aesthetic one may have deemed it hopeless--Parker, Aurora, Sheaffer...etc had all come out with sleek, curved models with metal caps and futuristic designs. And they sold well.But so did the 400. Maybe Germany wasn't ready for a change-maybe they didn't like the new aesthetic. For whatever reason, the 400 may have been mistaken as a pen behind its times...but underneath-a whole different story! The modern m400 came about as the fountain pen ice age ended, and a growing luxury market in the 80's spawned. Pelikan saw their chance, and took it--the Souveran was launched. Originally it looked the same as its predecessor, but over time, Pelikan (possibly trying to compete with Mont Blanc?) added trim and glitz to it. The 2012 Souveran is a very different pen from the 1950 Pelikan 400.
The modern Pelikan m400 comes in a beautiful presentation box, with a nice white leather case that looks like a letter. They used to ship with boring blue swirl boxes that were very standard. The "New Logo" pen boxes are much nicer. The vintage 400s CAN be found in boxes from time to time, but you would probably pay more if the box gave you a "NOS" feel to the pen. If you are giving a nice Pelikan as a gift, the modern "New Logo" ones win hands down. (Enough about the stupid boxes, Will ). Both pens are very light for most people, and the initial size may shatter your hopes..but when you uncap them, the pen is actually nicely sized--just as long and big as a Vintage Aurora 88, Parker 51, Midi sized Visconti pens...you get the idea.
Inked: The modern m400 pens have one advantage in this category--the colors and brands of inks that you can safely use is higher. The vintage 400 has a metal inner cap that might be corrosive with some boutique inks, and the celluloid binde might stain if you fill it with a not-washable ink and let it sit. Past this, they are very similar.
m400:10/10 (unless a white tortoise) 400:8/10
Filled: (yes, there is a difference between inked and filled in my book ) Here is where the differences set in. You may think that all piston fillers are the same. I find that people seem to think, "If it is a piston filler, it must score higher than a C/C (assuming it is smooth and doesn't break)." Ever wonder why Omas stopped making the 360 a piston filler? Because the new c/c version actually holds MORE ink!! Anyway, the modern m400 hold about .2 ml (or more) less ink than the vintage version. Bobo Olson on FPN has a great way of describing this. It is something to do with the "piston bore." The vintage 400's piston moves much farther up the barrel, drawing up more ink. Now, the m400 holds a great 1.8 ml, but why a company reproducing a pen would DECREASE the ink capacity of it is beyond me. The m400 pistons are smooth, but over time, they get stiff (in less than a year of daily use for me (daily use of the pen, that is)). The 400s might do this too, but so far not. If set up well, the 400's mechanism is just as smooth as a modern m400. (even smoother in my case.)
Clear advantage to the vintage here m400:8/10 400:9/10
Quality: This is a hard one. The m400 wins in terms of precise manufacturing, but that isn't really a huge deal (the 400 is hardly shoddily made). The nibs on vintage pens are finicky, as well as the fillers (if unrestored). So I think if you are looking for a consistent quality, go with the modern pen. But if you are buying from Rick Propas, Richard Binder, David Isaacson...etc you should not worry about this topic
m400:9/10 (nibs can sometimes be a bit toothy--GREAT customer service saves it though) 400:7/10
Comfort: Automatic tie...gotcha!! The celluloid feels better in the hand, but the design (at least the parts that you can feel) is the same for both. The vintage 400 is a bit heavier when inked, but not enough to make a difference. The one other thing is posting--the m400 posts much deeper than the 400, so if you have to post the pen, you might want to go with (generally) the m400 because you won't feel the cap lip as much (the 400's cap lip is sharper too, possibly making it even more uncomfortable when posted.)
Written: Totally depends. The m400 gives you a guaranteed smooth feel, but there is NO flex, and very little spring at that. With the 400, you can get a world of nibs: OM semiflex, F Superflex...you get the idea. I'd say the one distinguishing thing is the flex option if you go vintage (though you can put a vintage flex nib on a modern m400). Both pens offer a wide range of sizes, from BBB to EF. My m400 is consistent, but my 400 is awesome, with a EF semiflex nib. Both have a nice wet flow. Again, this all varies if you go vintage. Another thing--the 400s have EBONITE feeds, which can make for a wetter, juicer flow. That is up for question though.
Design Notes: No, I am not wasting words here. There are differences in the design. Lets go from top to bottom: The vintage captop is a nice (but plain) stamped Pelikan logo, while the m400 (at least the new ones) have brushed gold captops; matching the rest of the trim on the pen. I think the m400 looks better, but the PVD (?) matte finish is prone to wearing and scratching even with a fingernail. Then there is the clip: the m400's clip is super precise, and has a curved tip, while the vintage pen's clip is more "plush" and has a sharper tip; more realistic to a real pelican's beak. The modern one looks better, and they both function the same way. Now, if you are going to unscrew the cap in order to clean the inner part of it, the modern pen has an injection molding port to worry about when you put the clip back on--you must get the clip right in the correct tiny notches or else it will look bent. The vintage 400 has a smooth top, so it is much easier to put the clip back on properly. Moving downward, the cap rings are the same, (minus the mini one on the vintage though). The Vintage 400 overall is a little less "glam" and a bit more of a practical, well made, simple pen. The lack of trim at the nib section is another plus--no worries about corrosion!
m400: 8/10 400:9/10
Price:The vintage 400 sells for $200 or less in most cases. With Pelikan's relentless price increases, the m400 now sells for ~$210. At non-FPN places, expect more around $220. I think the m400, even at $220, is a great deal. A gold nibbed piston filler with lots of nib options, great customer service, and a nice design means that a Souveran series pen is a must have for many pen folk. The Vintage 400, at $180 or less, is slightly better: you get flex, a larger ink capacity, and celluloid for less. If you are worried about nib creep (NOT inky fingers, but ink ON the nib), the m400 is your pen. There is considerably less of it on the m400. Neither pen will leave you with inky fingers after uncapping though
m400: 8/10 400:9/10 (the 400 is really one of the best vintage user pens out there)
Conclusion: Vintage Vs. Modern? I think Vintage. Unless you want a glam pen, or you are giving a graduation gift (or something along those lines), the Vintage Pelikan 400 takes the victory.
OVERALL SCORES: Pelikan m400: