I love them both, but "Grail" does it for me. Much more Python-esque in my opinion, and a fantastically surreal plot through the whole thing. It also has more "classic" scenes in it, and blows me away every time I see it.
The book of it is also fantastic, featuring three different versions of the script, the final one including tons of material which was removed from the finished film, and even has a profit & loss statement from the accountant!