If I'm not mistaken (and I'm not 100% on this), but I've been told the VVT on the S14's is a bit tougher to mod. That and the S15 is newer and should in theory have fewer miles. More than likely most will have some pretty "hard" miles though. I've heard the "stock" S15 transmission is a bit weak, but I know NISMO makes a modded one for a mere $4500
http://www.rhdjapan.com/jdm-lo...20977 that addresses some of the OEM weaknesses. I personally like the whole getting an SR bellhousing and dropping in a KA transmission (cheap and easy to find).