"The SST pipe increases mid range pull and extends the over-rev considerably while sharpening the ... " - taken from fmfracing.com
You want a smoother, stronger hit from low to mid range for woods/technical riding,unless your really, really fast, or you like to clutch a whole lot. I would say look at the FMF Gnarly or PC Platnium 2 pipe if you do mostly woods riding
Spark arrestors are longer. That being said, some are longer than others. I think the longer it is the smoother power or better low end(or that's the claim), but I should think it would be minimal if any. However, you do need it to be legal....
Hope this helps.