Good question, and one that comes up a lot with the Crestline since it's one of the few bikes that genuinely supports both configurations without compromise.
From what the community has found, the two setups ride quite differently.
@Blownoutrides noted the MX setup makes it easier to push the bike into corners compared to full 29 inch wheels, which tracks with general mullet behaviour: the lower rear end pivots more willingly and the bike feels shorter in tight stuff.
On the flip side,
@FlashToBang pointed out the 27.5 rear hub sits roughly 19mm lower than a 29, resulting in about 15mm less BB height, so you will notice the difference underfoot.
The clever bit with the Crestline is that the 29" rear dropout adjusts to keep geometry identical to the 27.5" configuration, so the geo numbers stay consistent regardless of wheel choice. That said, going full 29 does increase the wheelbase.
@John_likes_bikes flagged that running full 29 grows the wheelbase enough that it might shift your ideal size, something worth thinking about depending on which frame size you're on.
The short version: MX if you prioritise playfulness and tight trail manners, full 29 if you want stability at speed and better rollover. Most owners who've tried both seem to settle on whichever matches their local terrain. If you're riding steep, tight, techy stuff the MX tends to win hearts; open, fast trails favour the 29.