I'd be a bit leery of playing PS2 DDR games on the PS3, to be honest. Although the mat says it's PS3 compatible, it doesn't say that it'll work with PS2 games. It's not something I'd assume without further research. I also don't think there are any PS3 DDR games out?
My wife is also into DDR, and we ended up just getting it for the xbox360. In general, I'd recommend the PS3 as a better system, but you'll probably have a better DDR experience on the 360. If your husband is set on a PS3, you may end up having better luck picking up a used PS2? They're probably dirt cheap, by now, and you'll get something that you know works. The PS2 still has the most number of DDR games out, and arguably some of the best, since that was the platform around at the peak of the DDR craze.