Can you swing both? Unlike the previous poster, I often use two (and sometimes three) 600EX-RT's off-camera flashes. I frequently light from one side with fill from the other. Here's an example of a 2-light shot (pair of 600's with the ST-E3 on-camera, flashes were in Lastolite 24" Ezyboxes):
EOS 1D X, EF 24-70mm f/2.8L
II USM @ 47mm, 1/250 s, f/6.3, ISO 400
The fill can be done with a reflector, too, but in this case I'd have missed the shot. He was turning his head from side to side as I was taking pictures, and there wouldn't have been time to keep switching the relative positions of flash and reflector. With the ST-E3, it took a couple of seconds to switch the relative flash power each time he moved.
Another situation where two off-camera flashes with radio triggering is nice is family gatherings. I put flashes in two corners of the room (on light stands, or a Justin clamp to a bookcase, etc., depending on furnishings), pointed at the ceiling. That way, no matter where I am in the room, the scene is lit with bounced (diffused) lighting.
If you must pick one, I'd get the 600 for the flexibility of having two flashes. You can set the on-camera 600 so it triggers slave(s) but doesn't fire itself. OTOH, the ST-E3 is smaller and lighter on the camera during use.