I'd say get whichever is cheapest per serving.
There shouldn't be any real difference between liquid and powder form since is supposed to be completely stable in liquid w/ no conversion to creatinine (the breakdown waste product).
Only thing about pills is that absorption may be slower if they're using gel capsules that would take some time to dissolve. Probably not more than a few minutes difference, though.
This is assuming there's no other active ingredients besides the creatine.