Well I dont have much experiance with Sound Ordinance so I cant talk to them very much. But they did get pretty good reviews.
As far as the Infinity vs the Fosgate? Id flip a coin. Both are very good manufatuers and both syspems will provide you with good sound. The Fosgate is a 12" while the Infinity is a 10". The 12 may give you a slightly deeper bass than the 10", but with the exta 50 watts the 10 has its probably a wash.
The Fosgate is your basic sub in a box. It has a seperate amp which takes up more space. The Infinity is a all-in-one on the otherhand. Just plug it in and go. It will take up less space and will be easier to hook up. If space is an issue (in a Mustang it usualy is), the Infinity is the winner. Another plus side is you also get a free install kit with the Infinity. Thats $50 right there.
And why not try to install it yourself? Thats half the fun. Its really not too difficult and you learn a lot