That's a tough one. All statistics are without stopping power.
MP5:
Kills in .2250000563 seconds without stopping power at point blank range
P90:
Kills in .2594594595 seconds without stopping power at point blank range
So the MP5 kills faster at close range.
Here's their hip-fire accuracy:
MP5:

P90:

I believe the P90 wins here, which could help even out the time to kill at close range.
Here's the sighted recoil:
MP5:

P90:

I suppose P90 is SLIGHTLY better here as well.
Clip Size:
P90 has 50 rounds in a clip, MP5 has 30. When you add into account the fact that the MP5 bullets do more damage: MP5 has enough for 10 kills, P90 has enough for 12.5. So, in theory, the P90 clip will last another kill or two longer than the MP5
Reload Time: P90 takes .5 seconds longer when partially empty, .2 seconds longer when empty.
I suppose the clip size cancels out the reload time, since you can use the clip longer and wait to reload until the time is right.However, the clip size will prove in handy if encountering multiple foes, so P90 has a slight advantage here in my opinion.
Iron Sights: MP5 has better sights.
I've always thought the MP5 was better, but now that I'm really looking hard at the numbers P90 looks like a better gun. It seems like everything the MP5 has, the P90 has a counter that makes it a tad better.
MP5 kill faster, P90 has better hip-fire.
MP5 reloads faster, P90 has a bigger clip and I believe the clip is more useful.
P90 has less recoil, MP5 has better sights. I believe lowered recoil is more useful than easy to use sights. You can get used to sights over time. Recoil is not going to change.
I guess my vote is for the P90. I know people will disagree with my interpretations, but the data I believe is accurate. When you disagree, try to focus on the data, not my interpretations of how it would make a difference in game.