mechanical difference, performance difference?

 

Pretty big difference. The ATTS is an electronic, active system. It uses sensors to detect wheel spin during high speed corning. It uses electronically activated clutches to slow down the wheel on the inside of the corner, and sends more of the engine's torque to the outside wheel to aid cornering. ATTS is not like traction control at all; it's meant as a cornering aid and won't help you in slow, bad traction conditions like rain or snow. Another downfall is it adds a good bit of weight to the car (IIRC 50~200 lbs). Most LSDs (like the Quaife ATB LSD) are a mechanical, passive system. It replaces the open differential in most cars and doesn't add any weight to the car. It has an internal gear system (some have clutch packs that require rebuilds) that automatically directs the engine's torque to the wheel that has less traction (i.e. the wheel that is spinning slower). It has no electronics and is purely mechanical. It can help in slippery conditions, but is especially great for flying out of sharp turns by putting power to the ground with both tires, instead of having your inside tire going up in smoke.
The Quaife ATB LSD is probably the best performance mod you can do to your car. It can't coexist with the ATTS system though. Power is useless if you can't get it to the ground, and I just can't wait until I get a Quaife.

 

thanks 4doorh22, my friend is going to buy a prelude soon and he is wondering what would be better at cornering, a base with upgraded springs/shocks, or stock prelude sh

 

Pretty much everyone here will tell you/him to get the base, and I would too.
I'd prefer a base 5th generation Prelude and add a Quaife.