I like to use a combination of both methods. The RotoZip tile bits are certainly effective, but they're very expensive and wear quickly. I cut as much of the "straight" with a diamond wheel in a 4-1/2" grinder as I can without overcutting, and finish off the corners with a RotoZip with a tile bit chucked up in it. It's not all that often I have to do this, anyhow, but a box cut into tile is a 200 dollar upcharge for me. With that in mind, I can take my time and do a superior job. I've followed electricians who have used a chisel and hammer on ceramic tile (no kidding), and they've left a few angry customers in their wake. Throw a cheap 29 dollar grinder, a 10 dollar diamond wheel, a 50 dolllar RotoZip, and a 5 dollar tile bit on the truck, and you'll be well prepared for any tile work. The diamond wheel in the grinder works real slick for cut-in boxes in brick too!