To start off, I have a Goerend tanny and could be happier with it or the service Dave offers. I live only 30 minutes from Dave's shop, so I am lucky to have such a close shop. I have spent some time in his shop and he has used my truck for some tunning on the 04. 5 and up trucks. I know that no matter what happens (or if) I have nothing to worry about, Dave will take care of it. He will also not try and over sell you on parts you won't need, if your building a everyday trailer tow-er or a all out sled puller.
Also I have talked with Clint Cannon,(ATS owner) on the phone and in person at some events that he has been to. And I really like the guy, I think that he has some very good products, a bit pricie. I think that he has received some bad PR about his trannys. Mainly (I think) because a person buying his trannys is paying top dollar for it and now a problem occurs, that person is going to let the whole world know about it. So I don't think Clint would be in the place he is by selling a bunch of junk.
So you have to decide who you feel would be best for your needs, I chose Dave because of the location,also I feel he'd do his best to take care of me down the road.
Just my 2 cents?