Sampras Fans might prefer Fed...
There will be as many different opinions as there are fans, of course, but some Sampras Fans might actually be rooting for Fed to break the record. If you're a Sampras fan, odds are good that you like serve and volley tennis and one handed backhands - think Becker, Edberg, McEnroe, Rafter... and you're probably pretty starved for this kind of tennis now. Even Wimbledon, with the slower grass, no longer displays much serve and volley tennis, and Fed often plays from the back court. But Fed is still the closest you're going to get.
So in a strange way, rooting for Fed might be the ultimate affirmation for a Sampras fan.
Personally, I generally root for contrast, because it makes for interesting tennis. The player personality matters to me too, of course, but Fed's "arrogance" doesn't really bug me, I think it's overblown. When huge serves dominated the game, and rallys were rare at wimbledon, I rooted for Agassi, Chang, and Courier, because they made matches more interesting. How things have changed - now I'd love to see a Sampras or Edberg, someone who would approach the net at every possible occasion. If someone could make that work against Nadal's remarkable diving spin, it would make for a very interesting match.