Hmmm S15 sr20det? Im assuming your new to this. I will help you.
Yes you can get an s15 sr20det with the 6-speed transmission, but swapping an s15 sr20det would require many modifications. The Speed sensor is in the rear.. the wiring is different, etc etc etc. The s15 sr20det is much more expensive then any other sr20det, and if your not getting the s15 notch top sr20det for just the 6-speed transmission then it would not be worth it. The s14 notch top sr20det is basically the same as the s15 sr20det. It just has a smaller turbo, 5 speed transmission, etc.
The s13 sr20det red top and black top from the nissan silvia coupe and the nissan 180sx (both only in japan) have the same amount of potential as the s14 and s15 sr20det. Even though the s14 and s15 sr20det have VVT, (same thing as honda's Vtec but with a different name) they have a "low port" design compared to the s13 sr20det's "high port design. Basically less restriction high flow vs VVT=no advantage=the same
All in all I can say that ALL sr20det engines have the same amount of potential.
This should be very helpful
S13/S14/S15 sr20det comparing
http://forums.********.com/zerothread?id=59346
http://www.heavythrottle.com/
More sr20det FAQ
http://www.srownersclub.com/jdm_engines.asp (same thing as heavythrottle.com)
comparing sr20det specifications (go to bottom of page )
http://www.xatracing.com/sr20det.htm