It depends on what else you want to see and when you want to go. Colors change from north to south - obviously - from high altitude to low and from inland to coast.

Also this year has been a very early and warm spring after a barely there winter. Id on;t know what this will do to leaves (turn sooner, turn later?).

I think you should pick an area in which you want to see other things and then add some color to it. I mean, you can;t look at leaves 8 hours a day.

Vermont is spectacular when the leaves change color. Rt.100 gives you a wonderful sampling. Lots of wow factor in VT.

The DATES of your visit will dictate where you go. As nytraveler already said, the colors change north to south. I'm just north of the NH/MA state line and we get peak colosr around the second week of October.

Plan on being in Rutland, VT on Columbus Day Wkend.

I agree with NYtraveler. Pick an area where there are other things you want to do. You might be unlucky to get a rainy week. You can get wine and cheese trail maps from the states. Some towns have foliage festivals. Check out some apple orchards that specialize in heirloom varieties.
If you fly into Boston and head to VT via I93 to I89, there's a nice view on I89 after you leave Concord. You'll know it. I like I93 heading north. As you get closer to the mountains, it just keeps getting prettier and prettier.
You can get a foliage cruise out of Portsmouth NH or boat ride on Lake Winnipesaukee. Or choose a hotel/inn with a great view because just sitting and looking is part of the fun.
If your adventuresome, maybe a half day kayak trip. Maybe see a bald eagle along a river.
Yankee Magazine's website usually has lots of pictures and suggested routes.

Any chance we'll see color along Maine coast and the White Mountains in mid September?

The coast is a warmer zone so it's very doubtful. You might see early color in the White Mountains. The earliest color is red swamp maples which can start changing very early so you could see color along I93 or I89 in NH. It's a limited area and isolated color. There's an occasional tree that will change early due to disease/old age but in general, no color in mid-Sept.

Mid September Maine coast NO. White Mountains - maybe but not peak and possibly not much. Very far north, almost to Canada would be your best bet in mid September. But I agree with those who say pick an area with other things you want to see/do because every year is different so if you need to make plans far in advance there is no way you will know. And last year was a dud for color and this year the weather is even stranger - could be wonderful or awful. The Maine coast will be lovely with or without 'color' (the leaves will be green and the ocean blue, those are colors) and mid Sept is a great time for that.