Have driven both I-75 & the Mountains & flown down. Everyone has pretty well pointed out the differences. The I-75 Book is invaluable if you go that route. I would never do the Mountains Route in the Winter. We took it Southbound early Dec. & ended up with some dicey weather once we were half-way.
The CAA TripTiks even warn you to avoid that Summersville/US19 route. Helpful Dis'ers had already well-warned us when I had posted about the route. But I was rather surprised to see it actually printed in the TripTik too! Apparently, as you can imagine, Ontario plates are a favourite target.
For us, 6 people with 1 driver & 4 kids, the I-75 Route is best for all reasons people have mentioned. Straight on it all the way, with the I-75 book you can avoid many traffic problems & stay entertained, countless possibilities for food, bathroom, & hotel whenever you decide to stop. The other route was a beautiful sight & I'm glad we saw it, but for us I-75 is just less stress. Of course, if we were veterans of the route like others on here perhaps we'd feel differently.