Well, I'm not working in a game store yet, but will (hopefully) be doing so soon.
I would ask them what their incentives are to buy the console. If it's because they'd been over a work colleagues for a few nights and played Halo, I probably wouldn't be reccommending them the PS3.
I would also ask them whether or not this is their (or their child's) first console. IMO, there'd be zero point in someone dropping £300 or so on a PS3/360 if they just can't get into gaming. So a DS would be less of a risk for them, financially.
Obviously, I'd bring in things like Xbox Live Vs Blu-ray, and other technical tradeoffs if they're interested in both the 360 and the PS3.
However, I'd also make sure that if they're buying the console for their child, that it's the console the child asked for. If li'l Jimmy has been over li'l Timmy's place and played his Wii, and asks for one for his birthday. L'il Jimmy may not be so happy with a PS2, just because it's cost effective.







