Your question is too vague. What year, what model/stage, etc... You should study the Saleen and Roush websites individually and you will understand what they are doing to them for the current model year.
If you are curious about a particular year, then tell us and I'm sure someone will chime in.
well someone that i work with said that roush takes the motor out and does a bunch of things to it. What, i dont know. He said he saw a show on it. I didn't think they did that much. Maybe I dont know $h!t about saleen...
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2003 black mach 1-
mac o/r H-pipe/ slp 2 mufflers
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To my knowledge, the Roush Stage 1 is an appearance package, stage 2 adds some bolt ons and suspension work, Stage 3 adds a supercharger, upgrades and more bolt ons. No where have I ever seen anything about Roush doing any motor work.
Saleen has the 3 valve which is bolt ons, appearance mods and suspension work, then the SC which has a supercharger and a few more upgrades and bolt ons, then the Extreme which has a worked over motor by Saleen. Roush has nothing even close to the Extreme.
You will get differing opinions on who likes what and why. Get what YOU like and enjoy it! Neither are a bad choice. Make sure you go and check them both out thoroughly. Of course, my signature shows my preference.