Here is my .02:
I am not a fan of going the custom route with the 700, you will never get the same $ out of it as you will invest. If you want a 700 style rifle sell the one you have and build off a custom action like the new Defiance, Big Horn, or a Surgeon 591. There will be those who will tell you its not worth it for the custom action but take it from someone who has many of both, it is and you will see as soon as you try it. The other benifit is a custome action build will hold its resale much beter than a trued 700.
option 2 the AI: You say you have an AI already I would assume its a AE of the older varient. Facts are they are flat out kick ass, but the triggers do leave a little on the table although they are plenty servicable. Now if it were me (and it was last year) and I was in the market for an AI I would not settle for anything less than an AW or an AX, just for the simple fact of if I am going to do something I want to go all the way (the same as the trued 700 vs. the custom route). True there may be no tangiable difference between the AE and the AW/X for all intents and purposes but the latter will never leave you wandering...
option 3 the DTA: I too have a just a little time behind the DTA (in both 308 and 338) and for what they offer NOTHING can match them. I found the ergonomics quite suprising how easy it was to efficiently operate. With little to no time I was able to cycle the bolt with ease and maintain a good shooting position, the only thing I noticed was the brass had a tendency to hit my hand and flip back into the chamber. I think they balance better than any thing out there and not to mention the compact size for carry/manuver. I am blown away by the accuracy and repeatability of the system it is crazy accurate and repeatiable! The you can change calibers in like 60 seconds, WTF! I would like to try the new chassis with the adjustable cheek peice, looks like a big improvement. Now for the bad on the DTA, I think if you were unhappy with the trigger in the AI (which feels to me like a carrot breaking as opposed to the glass rod) I think you will be really unhappy with the DTA (inless they have improved them over the Gen1 rifles). They are plenty servicable and I was able to shoot scary good but it was a bit unpredictable. Some times it was crisp others not so much with some creep.
All that being said none of this is going to help you too much, b/c you are looking at 3 different style rifles. You need to decide what you are truely looking for. If you need the most reliable rifle out there then the AI is the ticket (although I would say 99% of those who own one will never need its durability). If you need the most versitile then the DTA hands down. Or if you just want a good solid rifle that you can customize to your liking to go and bang steel with and shoot a match ot two then the 700 style may be your best bet. I would say that any of these will give the same accuracy and sevice for the average guy, so you may need to ask your self some more questions.
But then again, I am just drunk and rambling on...