I have eaten at both, and have to go with Tusker House.
Y&Y is, at least for me, easily the most disappointing dining experence I have had in any WDW location. A menu that looks like it was thrown together with whatever they could get cheap from SYSCO - no rhyme or reason between dishes. The dishes I had are typical Landrys-Rainforest assembly line dreck. Give the customer slop, but give them lots of it, and they will be satisfied. It was just really really bad.
Tusker House on the other hand I enjoyed very much. I also like Shiraz with a lot of pepper in it, and the Goats do Roam Shriaz at Tusker House has beaucoup pepper in it. Food there was pretty good as well. It is a buffet, but then you know that going in, so you set your expectations to that level.
-dave