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I'd get a used one with more capacity. Search for "APC SUA750" on eBay... there's a series of refurbished units for $100 w/new batteries. :)
 
I can't find the power usage for your iMac, but off the top of my head I think your first choice would be good. UPSs are not designed to keep your computer on during long outages, but to cover you for short ones and allow you time to shut down for longer ones. If you aren't around and your iMac is sleeping, either UPS you mentioned should run the computer for a long time. To estimate times check your manual for power expenditure while running and while sleeping.

I've tried other brands, but prefer APC, so good choice there.

Don't bother hooking printers to the UPS, but I do have one on my Time Capsule and cable modem.

Mooch
 
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