The 3815 is good, but it doesnt offer any of the transmission control functions that the new 3865 has.
You never answered what your plans are for the truck? Cam/heads? If so, how aggressive? There are some pretty good cams out there that will run without custom tuning.
If you're interested in transmission control (controls shift points, pressure, firmness, just about anything you could dream of), you might consider upgrading to the Superchips Flashpaq 3865.
I personally have a Diablo Predator and it works great (came from a SC 3815 like you have). I made the switch because i needed custom tuning for my cam. The built-in tunes are a tad stronger in the DSP, but the SC 3865 more than makes up for it with their tranny control.
If you're looking to go all out with the custom tuning, datalogging, monitoring, etc, and you want an all-in-one package check out the diablo trinity.
Theres a lot of information here, so take it in slow. Ask any questions you want. To get the most accurate information possible, include everything you know about your truck and what your plans are for it