I recently did LGW->IST->BKK. LGW->IST J class was an economy class seat. There was a divider curtain between biz and Eco, and they didn't put anyone in the middle of the three seats, but it was literally the same seat as economy, which wasn't what the seat map had led me to believe.

When I complained about this on this forum in the TK section, everyone called me an idiot and asked what I'd expected from business class, which makes me think this is pretty common, rather than I got unlucky.

Food on the flight was excellent, but I've never been disappointed by the food on QR. Lounge at DOH had the edge for me, but the one at IST was nice enough (although packed).

I slightly preferred the 4hr/10hr format you get from changing in IST to 7/7 you get changing in DOH because it's a longer sleep.

Despite all this, had you asked two months ago, the answer would have been TK, as they were wet leasing a nifty jet airways plane for the IST->BKK route, and selling its first class suites as business class(!), if you had status, and these were awesome. Under the impression that ended a few days ago though, so in summary: QR you'll actually get the J seat you pay for, TK you may not do.