Fox because Fox has Aliens, Predator, True Lies, the Die Hard series, ID4, I Robot, Fight Club, X-Men 1-3.
More importantly, Fox actually releases their hit movies on high def. Of those I just listed, 7 are already on BD or coming. Paramount releases a lot of crap on high def and none of the big movies we all want. No Star Trek movies, no Gladiator, no Titanic, no Indiana Jones.
Comparing the output of the two studios, Fox/MGM puts out a ton more movies in the same amount of time as Paramount (compare the average per month for the two studios when actually releasing content).
Yeah, I'll take Fox's catalogue over Paramount's because I know Fox will actually release the movies I want to see on high def. Meanwhile, Paramount likes to wait for years to release the movies you really want to see.
Look at their DVD track record with movies like Titanic and Star Trek. Unfortunately, Gladiator WAS released quite quickly, but that was before Paramount conquered them and took over the reigns. I expect similar sluggishness in releases now in high def.