Not all UPS's are created equal. Cheaper ones use "stepped" sine wave inverters, while the more expensive ones have "pure" sine wave inverters. Pure sine wave units are obviously preferable, because of their better current regulation.
But what does this really all mean? In terms of electronics, pure sine wave is needed mostly for highly sensitive applications, such as medical equipment and server farms.
Common mantra among the computing world is that if you have a computer with an active PFC power supply (which Mac Pros are), you should splurge for pure sine wave, though this is highly debatable. "Pure" vs. "stepped" is only part of the equation. There are other factors that go into the design of a UPS, such as the quality of the inverter electronics (part-time vs. full-time), battery design, etc.
APC in particular separates their product lines into two main categories: BackUPS (consumer-grade "stepped" wave) and SmartUPS (commercial/industrial-grade "sine" wave). Aside from the obvious differences, SmartUPS models are built considerably better all around. And this is why a BackUPS 1500 costs under $200, while a SmartUPS 1500 approaches $400. I've actually used both on my 2008 Mac Pro with no glaring problems, but I did observe that the BackUPS was noisier and switched a few times over the course of a few months, while the SmartUPS never hiccuped.
But I'd always recommend buying the best UPS you can afford. If one can drop a few grand on a new Mac Pro and other expensive hardware, a few hundred on a quality UPS isn't hard to justify. There are also a number of vendors that sell refurbished SmartUPS models with brand new batteries for a fraction of the cost of new, so that's another thing to consider...