In the end, it's really going to depend on your budget and how much draw you'll be pulling in the event of an outage.
APC limits their "pure sine-wave" output models to their Smart-UPS line, which starts somewhere in the neighborhood of $450 for a suitable unit (1000VA and up). The Back-UPS and Back-UPS Pro models are all "stepped approximation".
If you're on a tight budget, stepped approximation is fine, as long as the UPS is a "line-interactive" type, which is far superior to a "standby" type. Line-interactive units use an autotransformer to condition the output voltage, allowing the UPS to cope with over- and under-voltage better. As a result, the UPS is more resistant to problems caused by brownouts and voltage spikes when switching over to battery power. Also, because the output voltage is conditioned by the transformer, it prolongs the life of the backup battery. You can get decent "line-interactive" models in the 1000-1500VA range for $150-250, generally.
There's also some good deals to be had buying refurbished. A lot of vendors sell good refurbished UPSes (APC, Tripplite, Eaton, etc.) with brand new batteries for a lot less than new.
Just avoid a "standby/offline" type UPS like the plague. Their inferior performance isn't worth the hassle over a standard surge protector.