I guess the point is whether he'd be averaging, say, 5 minutes a week at Spurs as opposed to 60 minutes a week at Saints. If those were the numbers he was looking at, I'd say his World Cup chances would be better if he came to Saints.
Problem is, of course, you can't know these things in advance (well, unless we offer him a contract similar to Guly's - with a clause guaranteeing him certain rights to play :-) )