GWB gets to rally his base if they drag this out, and he wins a long, hard-fought battle for social conservatives.
If the Dems have any brains, they'll whine a little and ASAP approve of Alito, then get to complain about all of the wonderful, constitutional stuff the Supreme Court does to save the nation. (please, please, please....)
They have to go for a filibuster, otherwise their rabid, knee-jerk leftist base will stage a revolt.
On the other hand, a filibuster attempt will almost certainly trigger the Constitutional optional.
They're caught between a rock and a hard place.