Well, GK can take Terminators as troops, without needing to use a special character, so that might be a selling point. By the time you buy a Land Raider, you could have bought twice as many Terminators, so if you're planning to make them your only troop choice, you're probably going to not want to do that.
Other than that... GK's seem like they have more limited support options than other armies. Off the top of my head, Orks can use Terminator-like troops, by taking Meganobs as troops with a Warboss, and they have quite a few fun support units. DA can take Termies as troops, but I think you need to take a Special Character in order to do that? They would have access to most, if not all, of the cooler support elements that Marines get access to. Another bonus is that they appear to be able to be much more flexible in their Terminator equipment. I think Space Wolves can also take dudes in Terminator armour as troops, but that needs a special character as well.
On the whole, I think I would suggest playing DA if you want to go all Termie troops. They would have a Codex designed for 6th, lots of fun new toys, and they're the originators of "Termies as Troops."
Of course, if it's the models that pull you in, then you gotta go that route.