Neither will void your warranty. MM is your best bet. The price only goes up from them. If you wanted some nasty ones that work just as good as MM (Note, not any better) you could go with Hans Racecraft SFCs. I believe these are the ones that you would need, unless you have a vert. http://www.hansracecraft.com/product...ustang-sfc.htm
As for the Steeda STB, well, it is ok, but, the 2 point didn't seem to do a whole hell of a lot on my car. I kind of wish I had gone with a different brand that had the firewall attachment as well.
"When I know more, I'll be forthcoming. Or I won't be forthcoming, and I'll be honestly deceptive." - Les Miles