Edit..... If you're G/F is blazing then we're gonna need proof. Pics or Ban!!!!
Ok I guess I get to chime in here....
First off If performance really counts then whether or not you look gay (Miata) or look like a fanboy bandwagoner (S13) won't mean anything.
S13 was my first car when I was 16, currently 23, and I've owned 4 of them and personally feel like it's the most competent platform to start with and maintain. I've owned and drifted an 86 and driven a few Miata's enough to beat up on them repeatedly.
The S13 chassis is more linear feeling to me because to the long wheelbase. It has more angle than a Miata but also weighs more. I feel that both cars lack in power department and need engine enhancements to truely be capable performers in the drifting/auto-x arena.
There are people who make the Miata work for drifting, but I feel with the lack of angle, short wheelbase, and size contraits on the width of the rear tire without overfenders it doesn't make it a likely choice. With everyone's budget in mind I think it's cheaper to mod the S13 into a capable machine for drifting.
If you were looking to mainly auto-x I would relent and say that a Miata is better than an S13. It's small and nimble and can seriously cut cones. If it's drifting you want, then I'd say S13.