I wouldn't trust any multimeter in that price class with Cat III/IV measurements. Most meters in Dave's $100 shootout didn't even appear to have enough protection for these circuits. USB/RS-232 is occasionally useful, but I would focus on the core features first. Between those two, I would probably go with the Uni-T because I've at least seen some experiences (search the forum) which seemed fairly positive, no detailed review or tear down, though. Negative was the slow continuity. No idea about the Mastech. Watching Dave's review in the $100 DMM shootout about the UT-71 might give you an idea what to expect, although this is a more expensive model.