Choose Acrylic for Most Jobs
Acrylic caulk is easy to use, easy to smooth out and easy to clean up. And since it offers reasonably good flexibility and adhesion, it's a good choice for most exterior gaps narrower than 1/4 in. Some acrylics are labeled 'acrylic latex.' With others, you have to read the fine print to find 'acrylic.' Price is a good guide to quality; bypass the cheap stuff and spend a little more per tube. Saving a few bucks on the seals that protect your home from water damage is a bad bargain.
In terms of performance, polyurethane is just plain better than acrylic caulk. It sticks better and retains its flexibility longer. The downside is that polyurethane is more difficult to apply neatly, harder to smooth out and tough to clean up. You have to mask off porous surfaces and keep mineral spirits handy. For some situations?concrete, roofing, gaps wider than 1/4 in.?polyurethane is definitely worth the hassle. But consider it for some standard jobs too. With all the time and effort a repair or redo would take, polyurethane might save you trouble in the long run.