I've seen 'em both in NYC and they are both fabulous, but different.
Spamalot is great for those who enjoy a more slapsticky humor. If you have seen the Grail movie once it is worth it. If you have seen the Grail movie a hundred times and can recite the lines, it is even better. GO.
On the other hand, if Mssr. Python doesn't do it for you, see Wicked. It is a wonderful show, there is not a bad seat in the theater, and the lighting is among the best I have seen (but I am an old stage hand so I notice things like that). The plot is imaginative, and the music is great. GO.
See 'em both. I did!
-Skip