Polar opposites. As far as contributions to the game, you have to say Seve. Starting with the runnerup at the 1976 Open Championship, he was the driving force behind bringing respectibility to the European Tour and competitiveness to the Ryder Cup.
Faldo had better success at the US Open and PGA because he was such a great ball striker. Ballesteros might be the worst driver of anybody with multiple majors, but what a magician he was getting out of trouble.
Seve 48 Euro wins, Faldo 29, each had 6 US wins. Seve won 20 points in 37 Ryder Cup matches, 11-2-2 when paired with JMO. Nick has the record for most cups played in (11) and was 23-19-4 overall.
I'm going to have to waffle and say Ballesteros was a more important figure because of his contributions, he was the pioneer, but Faldo may edge him as the better golfer.
Seve had the greater talent, Faldo the better record.
In their prime I'd take Faldo to have a better chance to win a tournament.
However if I was stuck in a head-high bunker during a torrential downpour, with a fried egg lie, 200 yards to the green, with a lake between the green and the bunker, I'd go with Seve.