As far as bass, if you like the bass on one you will like the bass on the other. Both have plenty or it and lots of impact. They have a slightly different SQ IMHO - the SF.5 is more user friendly (sounds great to you right away where as the UM2 takes a little time, but then sounds great) and I think a bit more consistant. I think the Um2 is more detailed and shows flaws more than the SF5.
I went to the UM2 from the SF5 based on comfort. Period. If the SF.5 fits your ears and is comfy and holds its seal, then IMHO one would be nuts to upgrade to the UM for $100+ more. It might be better, but not that much better.
i got the 5 pro yesterday and it sounds pretty good. i feel like i could use just a tad more bass, but i'm not complaining. so really no difference in bass between the two?
the fit is a little weird though. i've never had the suction effect to such a great degree. i'm a little more used to it today than i was yesterday. and my right ear canal is a little bigger than my left, but not big enough for the large tip.