Dr.Dase: You're not being analyitical enough.
The 5.7mm round is what I refer to as a 'jack of all trades, master of none' round. It has good armor piercing capabilities, but then again, so do some types of 9mm ammunition. Case in point, Arcanes, which are cone pointed and are fired at high velocity giving them, the ability to go though ballistic plates, engine blocks and etc. The 5.7mm alledgedly has decent stopping power, which the 9mm has, and when certain ammunition types are used, 9mm has pretty awesome stopping power out of the barrel of lets say your typical SMG with a 7-8 inch barrel.
The 5.7mm round has low recoil impulse. But only an insignificant amount in comparison to 115 grain 9mm ball. Not 'a lot less' as you say.
5.7mm subsonic may be quieter, but it's terminal ballistics are inferior to 9mm subsonic. Which by the way can be most selective given the huge variety of 9mm rounds out there.
Onto the platforms.
The P90 is shorter, yes. The MP5N/A5 varients are of variable length with sights close to the bore access for more precise shot placement. The P90's collimator sight is higher up on the bore axis. Needless to say not an advantage.
It holds more ammunition, yes. 50 round and 100 rounds beta-C magazines are available for MP5s. The P90 on the other hand, has it's magazine capacity limited by magazine space in the receiver itself.
The P90 has it's weight distrobuted about the pistol grip, which makes for easier pointing, unlike the MP5, which is front heavy but that in turn makes recoil easier to control. The MP5 has a low center of gravity due to the vertical magazine, which enhances stability. The P90's center of gravity is high up, which makes it unstable.
The P90 has an extremely shallow grip to frame angle. Which makes it compact, but makes it inversatile in handling. The MP5's grip angle makes for more comfortable handling and better point angles.
The P90 has a longer barrel yes, the MP5 fires a round that does not require a long barrel.
The P90 ejects brass downwards. While it makes things less complicated for left handed shooters being a bullpup, it also makes footing potentially hazardous for the user.
As you can see, the MP5 still has a lot going for it. So I must point out that a contest between the two is not as clearly defined as you might think. There are many reasons why the MP5 is still the dominant submachinegun the world over.
Mostly the fact because in features, execution and quality, it's simply better designed than anything else.