If you have a high efficiency PFC, a UPS of the proper design is a must. A good PFC power supply only does its job properly when fed a reasonable approximation of a sine wave.
On-line (double conversion) UPS units output a sine wave at all times, but are inefficient and usually very expensive.
A stand-by UPS may or may not have a switching time fast enough for a PFC power supply, and will almost never supply a reasonable approximation of a sine wave when running on battery (I don't recall ever seeing a design that did, anyway). Cheap as chips, so to speak, but you buy a UPS to keep you running through a brown-out or black-out, and everything works just fine EXCEPT the computer you were trying to keep running. They will keep running non-PFC power supplies, though, no problem at all.
A line-interactive has close to the efficiency of stand-by UPS, but with the benefit of always having the inverter/converter always connected to the output, so the response time should never be slower than what a good PFC power supply requires, and depending on the converter logic can output a good sine wave approximation. Unfortunately, many UPS units output what might be generously referred to as a "stepped sine wave" which is frequently closer to a square wave than a sine wave, so your cheap as chips power supply computers keep running fine, but your nice, shiny PFC power supply computers shut down.
Unfortunately, I live in China where lots of line interactive units that output reasonable sine waves are made, but the only way to buy them appears to be quantity 500+. I have yet to find a unit sold at retail in Zhongguancun (Beijing), Taobao, or 360buy that gives a good sine wave output without being an on-line unit. And, no, APC units- stand-by or line-interactive- do not produce a good enough sine wave for my FSP AU-400 (80+ Gold, happily reliable on Beijing wall power now for almost a year after my OCZ 500 caught fire... twice, fun story!) to keep it from shutting down, though I respect that this power supply may be more finicky than most. Cyberpower makes a nice unit (eg CP1500PFCLCD), but I can't get a 220V unit imported to China for anything close to a reasonable price, and carrying a 110V unit back from the USA with me would require using an external transformer defeating much of the value of having a line-interactive.