From what I can see, the biggest difference is in price. Many times, the CyberPower seems to be a tad (anywhere from 10-20% cheaper, especially on the larger units--500VA or more) than the APC units.
I've never really used Tripplite myself, and haven't really known anyone who has (at least, their UPS systems).
I suppose if both the warranties are the same, I might just as well go with whichever one is cheaper (APC or CyberPower). I've heard both good and bad about both, so that sort doesn't help any in my decision. As we had said, as long as it can shut down the computer within a few minutes, and at least run the computer for 5 minutes to shutdown, then that's all I need.
I'm doing this mainly for my server computer, which happens to be an Intel Atom ITX board with two hard drives (draws, at most, 40 watts from the wall at full load), so I'm thinking that a 350VA or 500VA should do the job nicely. I originally was going to put other stuff on the UPS, but then realized that a good 1000VA UPS will be a bit more than I care to spend, and the important thing for me is that my server shuts down normally. The cable modem and router don't really need to function when the power goes out.
I guess in reality, it is similar to deciding between a Ford and a Chevy. Both have good points, and bad points. They have similar warranties, and similar safety features////