There is quite a difference with the M105+M205 vs UC+UP. In a body shop environment there is no question - M105/M205 all the way as UC and UP are not body shop safe.
For the casual user working via D/A, UC is much more user friendly in that it creates far less dust and offers a longer buffing cycle. Remember, M105 was primarily designed for use with a rotary buffer and wool pad to remove sanding marks from fresh paint. If you're working by hand it's sort of a toss up, but M105 can provide more cut than UC this way.
M205 vs UP is a slightly more closer call, though the M205 IS DEF more aggressive than UP, so you will achieve better correction(with the correct pad) that with UP. For the very same reason the M205 has a learning curve, though for the casual user the difference might not be visible when using by hand.