I tried buying tickets on this exact route/airline last fall, what a rip! In my opinion the LH plane is the best (a brand new Airbus 340), MH has older planes and charges the same amount, and I dont think the TG planes on this route are flagship products either. Ultimately I tried several Thai travel agents and they were not able to beat Expedia by more than a couple of $$. We had heard over and over again how a Thai travel agent would be able to get us dirt cheap fares b/c this was ex-bkk. We even were planning to have the tickets dropped at our hotel. Ultimately we paid a little more than the (seemingly expensive) coach fare and went for the nice lie flat seat. Boy that was nice! I think we tried all the various LH country sites and ultimately found one that had about a $40 better deal and we bought there.
We decided against Air Asia because we were connecting to MH in KUL and werent sure if Air Asia could check our bags thru. I'd just take them if you are trying to save $$.