From what I've read on here, the best bet is a Schroth Rallye 3 and any harness has ASM technology. ASM is an anti-submarine mechanism that keeps you from sliding under the belts in a front-end collision.
Basically what the issue is here is your stock belts allow you to move to the side if the car rolls on its roof, and the locking mechanism lets out a little slack first and keeps you from sliding under the bottom belt in a front collision. If you get a traditional 4 or 5-point, those belts don't allow your body to move out of the way properly in an incident. I read it somewhere on here, but basically you don't use a harness if you don't have a fixed back seat, and you don't have a fixed back seat unless you have a cage.
I'm not sure about SCCA rules (albeit I SHOULD since I've been getting more and more into it), but if you're just autocrossing then I honestly can't see how any type of harness would hurt you because I just don't see someone on their roof or into a wall during an autocross. However, NEVER use a harness (other than a Schroth-type ASM) on the street. I even read on here how you really shouldn't drive a caged car on the street without a helmet or else your head will hit that solid metal cage instead or interior plastic.
This is a pretty good read:
http://www.ft86club.com/forums/showthread.php?t=17216