Silver, despite the overwhelming endorsement on the 3, remember that ultimately you're the one driving the car day in, day out. So it's what *YOU* like better, not what everyone else likes better. Both are good cars, and in terms of reliability, you aren't gonna have very many problems with either one. If you find the Sentra more comfy, by all means, go for that.
The SE-R sports a 2.5L 4-banger, while the S sports a 2.0L 4-banger. You'll have to check Nissan's website for fuel economy figures. The SE-R also comes with a lot of other sporty goodies as well. But check Nissan's website for details on the differences between the two.
-Lik