I like both, but I prefer Kovu over Simba. Simbas has never been my top favourite character, though it`s hard to explain why.
The first time I watched SP, I fell instantly in love with Kovu. I find him to be a very interesting and complex character (not saying anything against Simba here) and I simply like him better than Simba. *shrugs*
I prefer Simbas voice over Kovus. Kovus design over Simbas.