The P90 is a bullpup PDW or personal defence weapon. It's magazine capacity is 50 5.7×28mm rounds, which are very small but extremely fast. Because it is a bullpup, its weight is more towards the back which lets it handle recoil much better. The P90 is manufactured by FN Herstal. Its ammo is a bit more uncommon than the MP5, but it is very effective.
The MP5 can typically hold 25 or 30 9mm rounds, which pack a bit more punch than the P90 at the cost of magazine capacity. It's manufactured by Heckler and Koch (pronounced coke) which is one of the best firearms manufacturers out there. They're made in Germany, which means they will last forever and are practically made.
In terms of which is better:
The MP5 has larger rounds, but a much longer profile, so the SG teams would probably use these while NOT exploring, and if there was a large field battle or something going on. Actually, they would probably use M4s and MG-36s in a field battle, along with mortars and other goodies.
The P90 is great for what the SG teams do, which is exploring close quarter areas and travelling light. 3 clips X 50 rounds = 150 rounds, which is quite a bit of ammunition.
All in all, both weapons are for close quarters, but the P90 is much more compact. The MP5 packs a heavier punch though, with 9mm rounds.
All depends on what you want to do. (this info was dug from my head)