We've done both and on balance I'd vote for Grandstand.
Love Congress Park...love the five-minute walk to Marketplace...and I will say that a room in Congress Park that has a view of DTD is special, but one on the other side of the building that overlooks the parking lot not so much! We've had both and while views aren't a big deal to us, the difference was really really noticeable!
Also love Grandstand, and it's 3-minute walk to the main pool (or step out of the building to the Grandstand pool/splash-and-play area), and first bus stop status...
It's also only a five minute walk to West Side...if you're eating at Wolfgang Puck's (one of our favorite restaurants), it's actually closer (by a lot) than from Congress Park. Ditto for DisneyQuest, Cirque, or the movies...Pleasure Island is six of one half a dozen of the other. Marketplace is significantly closer to Congress Park, but is where the boats drop you off and the boat ride starts just a short way from Grandstand...since a boat ride is an "outing" in and of itself, as well as "transportation," that makes up for a lot of the distance/convenience that you "lose" if you don't stay at Congress Park --
And, if you like to walk for exercise, as I do, from Grandstand there's a gorgeous walk around part of the golf course over to Old Key West (it's about a mile) that's really nice...of course, you could walk to the starting point of that path from Congress Park, too...it would just add another quarter or third of a mile to the walk...
At the end of a day in the parks (or anywhere), and you just want a snack (and didn't stock your kitchen/kitechnette), the walk from Congress Park to Artists Palette can seem long, even tho it's only 1/4-1/3 of a mile...it's just across the way from Grandstand. And getting on and off the bus first turned out to be a bigger deal than I thought it would.
So, we'd vote for Grandstand, altho would be perfectly content at Congress Park if Grandstand weren't available...