UMP because it has RIS rails already.
It's MAJOR downside will be the mags. Like the P90 mags, they are hard to carry and require dedicated pouchs that take a lot of room on you.
My plate carrier carry 4 P90 mags flat on my chest. I could carry a total of 8 if I double stack them, but I prefer the lean profile to fit into small spaces better.
Carrying those mags in drop leg is easy in CQB, when you are not expected to crawl much, but for outside use, they are very cumbersome.
And if you play with a Hicap, your a pussy...
MP5 stricly for it's mags. You can carry a shitload of mags, easely close to 10 on you and still have room for other types of pouch to carry other battle necessitys.
The UMP have a Gen3 gearbox I think... MP5 have a gen2... That's should be a major factor only if you plan on running it at 350fps and up and play in cold weather.
So, it all depend on your style of play and what you REALLY require. Look is a last but final desision point IMO, as it's only important if you hate the thing in the end or not.