I just stayed at the Aerotel, which is notable as the only airside hotel in the world with a swimming pool. I was actually quite pleased with my room and stay. Had awesome runway views. That's in T1, but as noted above, T1, T2, and T3 are all connected airside so it doesn't matter much which terminal you choose. The only weak point IMO was the fitness center. If that's important you might want to choose Crowne Plaza, or even head into Singapore, which is only a 20-30 minute taxi ride away.