Complex.
But that's assuming it's a good leveling system, if it's a bad system, for example with uninteresting upgrades that don't really feel like they make much difference, then a simple system is better.
In a good complex system, I can make a creative build for a character that really feels like it's my own character, and that I'm playing the game my own way. Or it feels like I could find some exploit to build an overly powerful character and there's a satisfaction that goes with coming up with some clever way to do that. Simple doesn't feel like much of anything, it's like you're experiencing exactly what everyone else does with no variation, leveling up doesn't seem very special in that scenario.