After much looking I finally found a floor that I really like (looks wise), but it only comes in 5/16" thickness. The guy at the flooring store says this may actually be more stable than 3/4" due to less shrinkage. (I live in MI). At any rate, I'm kinda I'm leary with less than 3/4" and feel like this is basically "cheaping out" on a low quality floor, but I really like the look of the floor. How big of a deal is thickness? Am I making a mistake going with a 5/16" floor or am I overthinking this?

It's the BR-111 Brazilian Hickory that I'm looking at.

I would say if you like the look go for it. They have a tough finish, and the Braz. Hickory is very dense. I would advise against their engineered.
As far as stability/ shrinkage compared to 3/4". I dont see where that's an issue. You still need to acclimate under normal living conditions, keeping your RH between Manuf. range.