It depends on the length of the flight. Up to 5 hours I can fly economy and not feel shattered at the other end, although this depends on whether I get an extra legroom seat / empty seat next to me which can even increase endurance to 7 hours. In that case, I’d fly economy and splurge with the hotel.
If it’s more than 5-7 hours, I’d fly Business and stay in a decent 3/4 star rather than super luxury.
How I start and finish the trip often influences how much I enjoy it. So no point staying in luxury if I spend the trip dreading the flight home, I guess.