For only a difference of 150 Euro I would not consider Royal Nepal Air.
Thai is far superior. Just indeed try to make sure that the flight is not overbooked, since this is around the peak season for Nepal.

Another important issue is that the Royal Nepal Air flight back to Bangkok leaves at 8.30 a.m. In Kathmandu you have to be at the airport 3 hours in advance, even if you fly Business or First. This means at 5.30 a.m.!!! I only fly at civilzed hours, for which the TG flights perfectly qualify.

Hendrik