from the two choices. MP5 over the P90 just because of the availability of aftermarket parts available, they have low--med-high price ranges to fit your wallet. if you go for the collapsible stock version (a5) you will have a smaller profile overall.
The MP5 doesn't feel like holding onto a plastic box compared to the P90. though the design of the P90 is comfortable. the P90 mag design alone are enough to deter me from purchasing or using one for cqb
MP5 mags are easily accommodated but id probably get a nice low profile chest rig with dedicated mp5 mags or open top shingles for quick reload
If you had a choice with what is new out there. I put my money on the KAC PDW AEG if you can get your hands on one. but it is a tad heavier than a full metal MP5. Or maybe wait for the release of the Airsoft GI - Magpul PTS at SHOT Show 2011 - YouTube