I'd really suggest going the DSLR route. You'll find flash sync easier (it's possible to use off-camera strobes with the S3 via manual control of bot h your flash and the slaves, but you'll have a significant learning curve going to multiple flash regardless of how you try, and E-TTL on an EOS will be much easier to cope with). Noise will be better on the larger sensors of DSLRs. DoF control's easier with a larger format sensor because of the longer focal length of the lens. The optical reflex viewfinder of the SLR gives you a much better view of what you're actually going to capture than do the EVF or LCD on the S3, since the optical finder's not pixel-limited and responds quickly. The DSLR has much less lag time; when you're photographing squirming children or pets, that matters. And you'll find that manually zooming the lens gives you much better control of your framing than the motorized zoom on the S3.
Those are a few of the practical advantages of the DSLR. There's also the marketing issue of your clients seeing you with a P&S rather than a big, "pro-looking" SLR. And don't underestimate that point's importance, in their eyes at least.


