Lahaina certainly has more things to do in terms of touristy stuff. When I was in Maui last Oct I found myself going there for a lot things. I was staying in Wailea and spent much of my relaxation time in Kihei itself. It is truly a gorgeous place to chill out at.
You should definitely rent a car if you weren't thinking of it already. The road to hana, word of advice, make sure you're on full guard going to the black sands beach. There are a ton of ass clowns that don't understand what road signs mean.
Bring warm close if you do the Haleakala sunrise, I froze my ass off up there