In the builds I had a chance to play, Ridge Racer DS was far superior in the gameplay department. Urban Asphalt GT looks better graphically but was incredibly slow in comparison. It felt like I was driving a go-kart.
The short: steering in Ridge Racer DS was loads better with the thumb nub. Once you really understand what you're doing it's not that bad. It took about two full laps before I started to understand how it worked, and the rest was cool. Both of those racing games need work, but Ridge Racer was actually pretty fun in the build I played, which is more than I can say for Urban Asphalt GT.