The several comments about the cost differential between the "line interactive" versus "on line" UPSs spurred me to do some quick research, and I noticed several interesting things:
1) APC appears to have been bought out by Schnieder -- the same people who bought Square D. I have no idea what the practical effect of that change might be.
2) APC / Schneider now has two flavors of the SmartUPS: a traditional line-interactive series, and an on-line series that appears to be targeted directly at Liebert.
3) APC / Schneider seems to have learned a few things: I notice that many of their new models now have battery trays, instead of simply stuffing batteries into the finger-eating maw of sheet metal.
4) The price differential isn't as great as I had supposed: for a 1500VA model in a rack-mount profile (but without accessory rails), CDW gets $685 for a line-interactive APC. The newer on-line APC runs $1,107, and the Liebert GXT3 is $1,033. So the premium for an on-line unit seems to be on the order of 50%.
As a rule, the reason I deploy a UPS is because the cost to repair a piece of equipment, or the consequence of its downtime, is at least as high as the cost of the UPS. It's not only important that the UPS work dependably, but that it not itself cause the very sorts of problems we bought it to prevent. Our experience has been that in general, the APCs have a practical working life on the order of 8-10 years, and go through batteries every 2-5 years. Our oldest Lieberts are still in service after something like 15 years, and seem to get 4-6 years on a set of batteries. In fact, we have had only one Liebert fail (a casualty of a close lightning strike... but though it died, it protected the three $20K graphics generators). But we regularly have APCs spontaneously drop dead -- usually taking down power to whatever they were feeding. It's somewhat empirical, but I figure that the lower mortality rate and longer battery life means that we actually spend less overall on the Lieberts now.