We did this tour this past August and loved it1 It's a lot of fun for the kids and there are some fun surprises.
Either day will work...the best would count more on your family. Will everyone be anxious to get going on rides and such the first day there and not have patience to wait until after the tour to hit their favorite rides? If so go the last day. Or maybe your prefer a chance to have some more relaxed fun the beginning of your first day (you do get to go on 1 ride during tour and have a couple of character M&G) and then book FP+ for afterwards and then hit all your favorites and be a bit more commando the last MK day when you know your way around a bit better.
It really all depends upon your family's travel preferences. Just a bit of advice (if you don't want a spoiler don't read on) ...if you do the tour the last MK day, you may not want to use one of your FP+ or time on previous day to meet the talking Mickey on Main Street...because if the tour is as it has been, you will meet him at end of tour.
Have a great trip and I hope you guys enjoy the tour as much as we did.