morrowind rebirth 1.5 or morrowind rehaul 2.0, which one is better?

They are very diffrent.

Rebirth will transform the world, for example new creatures, new weapons, rework town layouts etc. The overhaul will transform the graphics & sound. You'll be able to use both, but it's a bit tricky to make em work togheter nicely.

My advice is usually to install MGSO then Rebirth + deactivate grass.