To be very honest, I sometimes favor the younger one. My two kids have very different personalities. My older daughter (5 yrs) is a bit spoiled (my fault), and she's a little more difficult to deal with. She often gets hysterical over very little things, and that drives me nuts some times. Whereas my younger one (2.5 yr boy) has a very pleasant personality, very accommodating, and much more easy going than his sister.
This is all in my thoughts only, and I try as hard as I can to not show it. Once a child realizes that you favor the other sibling, he/she will start to hold a grudge and that is really not healthy for a kid.