F is F, and even mid-range F beats best-in-class J.
If you get can on TG's A380 or a re-furbed 747 you'll have a great time (the A346 F is OK but nothing to write home about these days). The service will be anywhere between good and off-the-scale-great. The TG catering in F will be very good, but not in the class of SQ F.
As others have said, the ground service in BKK is fabulous - escorts, nice lounge, full-body massage etc.
Go TG F. Don't think twice.