I can only speak to Star Alliance routings, but in my experience it is shorter and easier to fly east via Europe taking an evening flight to LHR or FRA (on UA or LH) and an afternoon flight on TG arriving BKK ~ 06:30. Returning you leave late at night and arrive in the U.S.A. in the early/mid afternoon. Returning takes a bit more travel time, flying west vs. east, due to schedules so total round-trip elapsed travel time is roughly equivalent. It all comes down to individual requirements and preferences.