Get the AK. I'd get the CYMA AKS74U if I wanted a small one. Get AV'd and they're on for $360 in the classifieds. There's more AK things around than you can shake a stick at...mags, batteries, rigs, etc...
UMP's are as long as a MP5 but much bulkier. Unless you know you can get/find UMP midcaps...forget it. Hicaps suck and I was stuck with a Star UMP that I couldn't get mags for. Plus...the mags are stupid big. Almost twice as thick as an MP5 mag and straight. Pain in the ass unless your rig is setup to carry that type (i.e. P90/UMP) mags.