Both games are looking awesome, but they're both fairly different styles of games.
Never played a Test Drive game, but from what I've seen of TDU1 and 2, it looks kinda like a Burnout Paradise type game but with more depth and content.
Shift 2 on the other hand is basically a sim, along the lines of GT and Forza. Less depth and class than GT and even Forza, but still looks very cool. And if you're a PC gamer, it's about the closest thing to a GT or Forza for PC that I can think of.
I'm personally planning on getting both. They look fantastic.