Go CES dude....i recently ordered one for my s15.
To make a long story short....the CES unit has been bread with quality and performance in mind and has proven dyno results...just search the many forums on the net and u will see what i mean.
If your like me, i like to do the job once and do it right....thats why i went for the CES unit, i don't see any point in stuffing around with the cheaper copies....waste of money in my opinion.
I must say the CES unit is still quite expensive but hey, that is the price you pay for performance!