Just taking another look at APC S15, I see it weighs quite a lot less than the Belkin, only 47 pounds. The Belkin weighs 77 pounds, 30 pounds more. I think about 15 pounds of that is a lighter and lower power battery. The remaining 15 pound difference probably just lighter weight stuff, including lighter weight transformer. Although this contributes little to the weight, the Belkin claims 8000 joules of surge supression, whereas the APC claims only 4080 joules.
Anyway, given that the Belkin does not impress me with UPS overbuilding, it would be hard to believe the even lighter weight APC would impress me more. And I know it's hard to judge these things from appearances, though I still do.
Did I mention that not only is the AVU1500 being discontinued, Belkin announced it will be leaving the UPS market. That does not sound good, though it might be that they simply couldn't compete against the marketing clout of Monster and APC. Or they could have had too many product failures and decided to bail. If you read online reviews of the AVU1500, it seems that one of the most common complaints is that the unit is essentially "dead" when first powered on. You have to allow 12 hours of charging before you can get the blue information screen. But people should know that, all my units came marked with a big warning label "CHARGE 12 HOURS BEFORE USE". So if that's the only complaint, it's really a user error.
I've also gleaned that if you discharge the battery all the way that you are back to the inital "charge only" condition with no operation allowed until there is sufficient charge in the battery. Hopefully that won't happen often, though it could also be considered a design flaw.
I've mentioned Monster in these comments without comparing specific UPS models. Monster does have a number of audio and home theater UPS, but at least the ones I've looked at so far are only stepped-approximation-to-sinewave generators, which are generally considered inferior . If Monster does have a sinewave generating UPS, you can bet it will cost more than similar products from APC and Belkin.
I already feel a big guilty about buying 3 of these UPS's. It seemed like a no-brainer when they were selling for $349 at buy.com this summer. But just after I got my first one, the price shot up to $500, so I got two units around $500, and now the price is up to $770, right about the same as the APC S15.
So I've put about $1349 into power conditioning and protection instead of music or more cutting edge audio equipment this month. (But the month is not over yet...)
I need 3 because I need one for the main living room system, one for the kitchen system, and one for the computer room system. I decided not to get one for the bedroom system, because the latter is only switched on when in use and is already protected by a nice surge/filter unit.
OTOH, I think there is audible improvement comparable or better than expenditures in other areas, equipment protection, and that warm feeling of being safe in your own home next to your stereo.