those two are the most popular. i dont think any one is "better" than the other. they will both give you the same basic results, depending on who you get your tunes from. i know of a few other tuners, but you may not receive as much technical support as you would from these two.
From working @ DiabloSport for a year I know they put a lot of effort into their "canned" tunes in the Predator. They had one of the best Ford tuners in South Florida working for them by the name of Scott Beer. The dude knows his *****.
I've currently got the 93 Octane Performance tune flashed on my car and I can safely say that I notice a difference.
I can't say anything about the SCT since all I can go by is what people on forums say.
Basically go with whatever tuner is predominantly used at your local dyno shops
I know around here its about a 50/50 split but I kinda do wish I went with the pred just so I could use the shop that is literally down the road from me but I am very happy with the sct