I've driven both, and own a '15 MDX.
To me there's no question the Honda is a better choice. Handling is sharper (almost as good as an Accord), acceleration is quicker, and it feels like it is better built overall.
The 3.5L VTEC engine is silky smooth, loves to rev, and sounds great (especially when you put your foot into it and the VTEC system switches the cam profile - the sound gets angrier and you get pushed back into your seat, lots of fun in an otherwise boring class of vehicle); no question it is a much better overall motor than the GM 3.6 (one of which puked oil on my pristine driveway when I had a rental Impala a couple years back). In fact, the Honda might be the best V6 currently on the market.
The interior isn't quite as fancy as the GMC, but the materials and switchgear quality are better, as is the overall fit and finish. If the Acadia follows the typical GM trajectory, the interior will present very nicely when brand-new but will quickly develop visible wear and numerous squeaks and rattles, then a little later stuff will start breaking.
As for the exterior, that's in the eye of the beholder. I personally find GMC (and Cadillac for that matter) trucks to be more pretentious and gauche than refined and sophisticated, and much prefer the cleaner, more refined look of the Pilot.
