If you can't get both gears + tuner at the same time, get the tuner first. It will give you a nice little bump - not overwhelming, but noticeable. And yes, it will come in handy for recalibrating the speedo when you do get gears. Also not to mention the diagnostic features of the device which imo make it an invaluable tool to have around. OhFiveStang hit the nail on the head when he said if you expect too much you'll be disappointed. It's not a 150 shot of N2O by any means.
As far as the AM/SCT Bama deal, you'll be hard pressed to find anybody who can give a legit negative review. They are out there, everybody has a bad day, but AM is very good in taking care of their customers should there be an issue. Read the product reviews on their site for whatever you're looking at; they allow all reviews to be posted, good or bad as long as it's not littered with profanity.
And you should really reconsider those 3.73's for 4.10's. I've come across countless people on the forums with 3.73's who wonder what life would be like with 4.10's. People with 4.10's never look back.