[Rocco]

Main goal for me is to win $ and for some reason I run good on Full Tilt and bad on Pokerstars. So, I play more tournaments on Tilt. Other than that there are many factors to consider...

Field sizes:
Stars Guaranteed tournaments draw way bigger fields than Tilt's dittos. Bigger fields = more donks, but it also means higher variance. You need to mix up between larger and smaller fields...

Software:
Stars client is more stable. Tilt seems to go down a bit too often, and if you happen to disconnect it takes way longer to reconnect to Tilt. Reconnecting to Stars is instant and the way it should be.

Support:
Never used them much, but when it comes to calling tournament directors over for final table negotiations, Stars react faster. Their overall support is generally considered much better if I'm not misinformed.

Tournament structures:
I really like Tilt's blind structures for some reason, but Stars changed to the better a couple of months ago, so it's pretty even.

Tournament availability:
I prefer tournament fees between $20-$100 and ususally start playing between 12.00 EST and 17.00 EST. During that span Tilt offers more interesting tournaments than Stars IMO. Overall it's even though, both sites offer many tournaments during the course of the day.

But, as Soulman said, playing on both sites is probably best.