Daryl - having sold a number of Ypipes over the years I can tell you the Stillen is better than the Cobra and will last a lot longer,
However, the best is the Motordyne XYZ pipe, you can have straight pipe, resonated or with HFC.
I'm ordering from both companies next week so feel free to let me know if I can help.
It would work out about the same price to order an XYZ with HFC and decats as it would to order HFCs and Ypipe so if it was me I would go Motordyne/decats everyday of the week
Chris