Since I have played both Wii Trauma Center titles...
Trauma Center: Second Opinion had a great story and was actually very engrossing. I thoroughly enjoyed it. Trauma Center: New Blood had a pitiful story compared to its predecessor, but was a great deal more difficult.
You say you felt compelled to get 100% on MP3? If that's the kinda thing you like to do, I would recommend Trauma Center: Second Opinion, mainly because the story is great. If you want a challenge, each operation gives you a rating. From lowest to highest, you can earn a C, B, A, S, or XS (Medical Prodigy) on each operation. Try to earn an XS on each operation for a real challenge.