Totally different roles for different teams.
What do they do differently? I think it'd be difficult to find two guys with more similar roles. Pure floor spacers. Ray drives the ball/attacks the rim more and has a bigger role in the clutch, but overall I wouldn't say "totally different."
I guess I misspoke. Their style of play is different is more accurate. Still, completely different teams.
Ray Allen has the luxury of playing off of James and Wade. He is an afterthought for defenses. Terry gets more attention. Switch the players on their respective teams, and you'd like see a different story. Ray would still be more efficient but less effective overall. Terry would likely be more efficient, but probably have the same averages in raw numbers.
That being said, sure Ray is playing better for certain. Terry has just been really bad on offense this year in strecthes. He has had some good games though. Terry is probably better on defense, as bad as he has been. Ray has killed Miami's defense this year when he is on the court.