Which are you thinking of and what are you looking for in a beach?

The Gili's have nice beaches and snorkelling with a number of lodging, eating options and other tourists. The South Lombok beaches are beautiful, empty of tourists (which some women find disconcerting safety wise ime). Often a bit too much surf for nice swimming. Only a couple "western standard" places to stay there - the Novotel Coralia, and http://www.heavenontheplanet.co.nz/ Sengiggi has the Sheraton but is a poor beach for swimming and the town itself was rather rundown (6/09, last time I was to any of these).

Plenty of nice beachs on Bali itself; for beach time I'd stick to Bali or head out to the Gili's on a fastboat. Wouldn't bother with Lombok this trip - as has been eluded too Indonesian Air travel has flexible schedules. So does the ferry, but the ferry's slower. Going from Kuta, Lombok to DPS airport via ferry and taxi takes 8-11 hrs total in my experience.