TG definitely has the more generous seat pitch (34" vs. 32", I believe). I've heard the service and the food are very good as well.
Inflight entertainment: no contest. It's SQ hands down. You get SQ's KrisWorld-- lots of options (60 movies for example, plus lots of TV programs and games), PTV -- versus films played on a main screen in TG economy.
I personally have always liked the food on SQ as well as the service level. An FA noticed that I had brought a small bottle of water with me, and instead of giving me water in one of those little plastic cups during a drinks run, she offered to fill up my bottle instead. This was on a full flight in economy.
Finally, IMO, transiting through Changi is a much more pleasant experience than transiting through Don Muang.
My very biased vote is for SQ, unless you're very tall, in which case the seat pitch may be of utmost importance to you. But as with all advice, YMMV.
