What are you getting the UPS for? If it's actually to drive a power amp during a power outage, you need a high-wattage unit (1500 VA or up would be ideal). If it's for power conditioning, then read on.
In general, a consumer-grade UPS will not power the line using the batteries. When the power goes out, they will turn on the batteries and inverter, and use a stepper function to simulate something that looks a little bit like a sine wave for AC. However, the signal has tons of harmonics, compared to pure AC power, so when you're on battery, expect 60 Hz (and harmonics like 120, 180, ...) buzz in your system.
High-grade UPS-es (typically costing thousands) will continually charge batteries, and continually pull power from the batteries, inverting to AC. They may also synthesize real sine waves using basically a really large oscillator. These guys can actually help a lot if you have very dirty power, because the rectifier and then the inverter basically act as a very, very steep filter for interference. That's not something you'll see from a $300 consumer unit, however.
A UPS may also be coupled with a power conditioning device, which is intended to remove harmonics from the incoming AC signal. This can help if you have really noisy power lines, and a weak power supply in any of your components. However, the fact that the consumer-grade devices come with power conditioning and UPS in the same unit, does not mean that it's an online, sine-wave, "active" UPS.
If what you want is clean power, I suggest focusing on some power conditioning unit. I've got an order in for the 1000 VA balanced power supply
with conditioner from Transcendent Sound, and I can let you know how it works once I build it (got the kit) and actually get my remodel done (so it'll be two months or so). Or you can go ask others in the same situation for recommendations.