I'm getting both so it doesn't matter to me. And both will make a lot of money. They're already talking ports of Diablo III to multiplatform. I'm willing to bet in the long run that Diablo III will sell more.
Max Payne has a wider appeal. Your average gamer(not talking about hardcore, but middle of the road) is more into guns and action over role playing. I'm sure most meat head gamers today would look at Diablo III and not see past the top down graphics. Hardcore gamers and average middle of the road gamers alike are interested in Max Payne 3.
If it was Halo 3 or Uncharted 2, instead of Diablo 3, people would be saying that it (X360/PS3 exclusive) would easily surpass MP3 on 3 platforms. But "no way PC exclusive can surpass MP3". Damn cliches and stereotypes.
Diablo 3. More people want to play this than Max Payne,and even people on this forum wouldnt care that much of Max if its still Remedy game.I never played Diablo games (not my cup of tea)but they got quite a reputation in gaming community but i did planned to buy MP3 but wait until GOTY edition so i dont have to bother with DLCs.