[Darklen]

UMP Pros:
-V3 gearbox (pretty much bullet proof)
- Rare (could be a con)

Cons:
- Mags hard to find
- Accessories hard to find
- stuck with mini battery

MP5 Pros:
- Mags are easiler than crap to find
- Accessories up the yinyang. Next to the Armalite series, probably the most accessorised aeg going.
- choice of mini or large battery depending on the stock (slither stock gets a mini, full stock gets a large.

Cons:
- Breakage prone V2 gearbox if overbuilt/over fired/used in very cold temps (-10°C)
- Seen all over, not rare

Not to mention there are more internal upgrades available for the MP5 series. Other than the usual V3 gearbox upgrades, I'm not sure what internals would be available for the UMP.