Games like these ARE a commitment either way, because of the length.
Both titles are somewhat bloated, however, Rebirth was more fun for me and really enjoyed my time with it.
That said, I can’t imagine playing 4 hours of Remake saying “this sucks” and then being excited by Rebirth. So, if you want to enjoy Rebirth to the max, I do think a Remake play-through first is a must. (Remake is 30-40 hours + Rebirth 90+110 hours).
So, with the little I know, I recommend you play FFXVI first. The game is around 50-60 hours, but the first 3-5 hours are pretty cool and cutscene/story heavy. So, if you only play one game for 2-3 hours, the. FFXVI might make a better impression with the spectacle.
Afterwards, even if you skip Remake, Rebirth’s sense of scale might be more pronounced after playing the sometimes claustrophobic FFXVI.