I definitely prefer Leia. While Harrison Ford is a great actor and Han has admittedly a lot of great lines, his macholike behavior (especially in ANH) never sat very well with me. Not to mention that he's unnecessarily rude towards especially Chewie and Threepio on several occasions.
Leia, while sometimes being a bit bossy, also often shows an empathic side of hers.
I like both characters in the end, but I definitely prefer Leia