I think you've made the best chioce.  The Smokies have very good color, but
the mountains are not criss-crossed with roads the way New England is.  I am
partially disabled and am careful to explore where my odds are more
favorable to getting to good locations.  The west is a poor choice - there
are aspens and cottonwoods, but they will all be yellow (with rare
exceptions) and much fewer and farther between than the New England foliage.
I live in Las Vegas and get my fall color at the Grand Canyon north rim and
at Zion National Park.  There are some areas north of Zion in the Dixie
National Forest that I missed this year, but have put on my calendar for
next year.  Again - all aspen, but aspen in quantities I've never seen
anywhere else.

Colorado is another good option, but with a definate shift toward yellow
foliage.  Roadside vantage points are plentiful, but as the previous poster
advised, they have lots of altitude with that low-octane air, so reaching
vantage points away from the road is more work than low elevations.

New England is heavily covered with roads, and the more you get away from
the major routes and onto the backroads, the better.  The mountains aren't
rocky, but you aren't looking to shoot rocks.  The scenery will be filled
with colonial-era buildings, covered bridges, high-spire Congregational
churches (the Puritans), and farmland.  Great photo opportunites.

If you have several weeks available to you, aim for Columbus Day as the
typical peak time for southern Vermont, earlier to the north and later to
the south in the Berkshires.  You might also consider the Adirondacks in
upstate New York - very good color and various small lakes, too.  A good map
of the area should tell you whether the area has good road coverage.

When you want to shoot a particular subject, going for the best is always a
good idea, and New England is world famous for its fall color.  Some day I'm
gonna go back ...

Bob in Las Vegas

If it's a normal season, Smokies will come after New England, so if
you've got time you could probably do both. As I said in my other
response, you should start with Skyline Drive in Shenandoah National
Park and drive south until you reach the Blue Ridge Parkway. Following
the Parkway all the way south brings you into the Smokies.

Peak times at the north end can be a week or more earlier than at the
southern end, but the southern end has a higher elevation, so it might
be closer to simultaneous.

And New England color peaks a month or more before VA & NC. Which leaves
you time to hit upstate NY & PA on the way from New England to the
parkways and the Smokies.