As we discussed in your previous post UPS Units & "Pure" Sine Wave Outputs ?, the pure sine wave UPS units are really only necessary for high end audio or medical equipment. For desktop computers, the added expense won't be noticeable, or have any real benefit. Are they better, sure, but not necessarily worth the extra cost.
I have used Back-UPS and Smart-UPS models from APC, and they performed equally. I have read that APC added pure sine wave technology to the Smart-UPS line. I have also read that some end-users have complained about a high-pitched whine from those units, probably due to being connected to "dirty" power to begin with.
The real function of a UPS for computers is to prevent spikes/surges damaging your equipment in the event of a surge, drop, or outage of electricity, giving you time to properly shut down your computer and peripherals.