I concur with the above: take F if you are going to take premium class in Thai and if you have to take C, pray you get an aircraft w/ a new seat.
Some of the European routes like LHR seem to be regulars with the 747 new seat versions (refer to other threads for details), but the aircraft can easily be swapped last minute for an older version.

At the end of the day, it's all relative.
While an old C seat on TG is definetely sub-par, it's definitely better than
sitting in a Y seat for +10hrs. On a full-fare flight, I would never choose C in TG over any other carrier with a modern cabin. Chances are, however, that you will find a fare no other airline will match. x2 of Y (assuming the Y is deeply discounted..) for the updated C, in my opinion, is a bargain. For the old seat...it is not. But if the price does not bother you, you may even want to consider it...just don't expect anything from seats/IFE and you won't be dissapointed. If you can find a F fare that is reasonable, take the F they will treat you like royalty.