From the metal/service perspective I would go with SU (intra European leg in J is definitely better on SU than KL, TATL is better on DL if solo, SU if travelling with a companion; both are lie flat), but totally agree that 6 hours in SVO could be a nightmare - lounges are pretty bad there and there's not a lot to do. Also, unless you have a Russian visa I would recommend against transiting through Russian airports - you don't want to be stuck in a terminal in case of an irrop.