Besides the chemical makeup, the biggest difference between a two-part urethane and a two-part epoxy is that you HAVE to mix all of part A and all of part B when using a two-part urethane for it to reach critical mass and work. However, with most epoxies you can "ratio" them out in equal parts and they will still work. A two-part urethane if not fully mixed will essentially turn into what feels like a piece of latex rubber from a balloon with absolutely no adhesion to it whatsoever. Had a customer make that mistake once in a hospital under some beds trying to "save" the rest of the kit he didn't use for another job. Somewhere in my desk drawers I still have a piece of the adhesive. If I find it, I'll post a picture of what it looks like.
The other advantages to a two-part urethane over an epoxy is that it gives you a broader temperature range. At lower temps, urethanes don't become as brittle and susceptible to cracking. They also tend to have better moisture resistance both topically and from below than an epoxy.
Downside is they are a bit stiffer to trowel and they tend to have a horrible "green" grab (they're slow). So, you usually have to use a kneeler board or treat them like a wet set adhesive and stay off of them completely or you'll end up displacing the glue and leaving indentations in the floor. But, once they set, look out.
