Both should be amply hardy for your location. Pagoda dogwood, cornus alternifolia, is a Maine native with a very attractive tiered horizontal branching pattern. There are a number of selections with variegated leaves. Full intense sun may not be ideal since this is an understory or woods edge tree. None I have seen in full sun locations appear to be suffering, however. In such locations, attention to summer watering is probably important. Seven Sons Flower is a plant i have never much cared for, but that is a matter of personal taste. Those I have seen grow as large multistemmed shrubs. One could probably be trained into a tree form, but I think it's very likely you would be fighting root suckers given its natural inclinations. When first introduced, this tree/shrub was highly promoted. I have not seen any for sale in Maine or planted out in some years. This usually indicates something negative.

Shasta viburnum and cornus alternifolia are really two quite different plants. The first is a shrub with horizontal branching, great spring flowers and excellent autumn color. It is definitely not a tree, however. I'm not sure I see these two plants as being in any sort of popularity contest. A large cornus alternifolia in one of the variegated leaf forms is a tree of singular beauty. There really is nothing else like it.

I do not like heptacodium. It looked unkempt, threw branches every which way and frankly the flowers did not compensate for the mess. It is, as far as I'm concerned a one season plant. I see you are in zone 8 so I'd think you wouldn't have hardiness concerns about many trees. However, I know one poster on perennials, in a milder climate than mine, cherishes her heptacodium because it attracts migrating butterflies. I saw a photo of hers and it was more vigorous than mine in zone in 6a.

I'm not sure how fothergilla, itea, viburnums and hydrangeas do in your climate but I'd choose any one of these over heptacodium for fall color. The fothergilla and itea have lovely flowers and fragrance in spring, early summer and extraordinary fall color; the viburnum have fruit to feed the birds and delight our eye; and hydrangea have flowers, color, insect forage and some varieties'flowers and foliage range from pink, deep rose and red. You might think of any one of the dwarf varieties of these shrubs as complementary to your new tree.

Another tree you might consider is Stewartia which blooms in early july, has a handsome, muscular trunk and wonderful fall color. It is far less common than kousa. In my experience the kousa's fall color is nothing special. This is a good article about stewartia: http://awaytogarden.com/hot-plant-stewartia-an-ideal-small-tree/

Choice comes down to personal taste really. Kousa would be more upright growing and tree-like from the beginning. Cornus alternifolia would need some training. Both plants have variegated leaf forms which extend the season of interest as the bloom periods are short. Probably eventually too large for a really small area, but cornus controversa 'Argentea'/'Variegata' is another possibility. Stewartias are fine trees, especially once they mature enough for the mottled bark to appear, but each individual blossom lasts barely 24 hours, so the bloom is pretty underwhelming. There are also a number of relatively small growing magnolias to consider in white, pink, or yellow that would present no hardiness problems.

I grow both Cornus alternifolia (pagoda dogwood) and C. kousa and have a couple of smallish Heptacodium (7 sons). I love the all season appeal of the pagoda dogwood - it has panicles of small, creamy spring flowers (a bit like Queen Anne's lace) that turn into oodles of little summer fruit that slowly move from green through red to purple and end in deep purple, almost black. The birds love them, so the tree is really hopping in August, and the bright red berry stems remain for a bit after the fruit has been stripped. Mine in full sun had a peachy tone to the leaves in fall, a mix of pink, gold, and chartreuse that I really liked, though the ones in full shade aren't much for fall color. I also found the winter branch structure ornamental. They tend to grow as a multi-trunked tree IME, so if you want a single trunk you will have to do a bit of annual pruning. For me the only drawback is that the voles seem to love the roots of all dogwoods and have killed several including the largest of my Cornus alternifolia.

The Heptacodium will grow in a much more shrubby fashion, and as others have said, has really just the late season flowers and bracts to recommend it. I don't find the peeling bark ornamental and the form needs quite a bit of work to make it tree shaped.

Kousa dogwoods are lovely and don't seem quite as bothered by the voles in my garden, though they still have some vole issues. They aren't nearly as hardy as the Cornus alternifolia, given that my Kousa is still working on leafing out after having all its above the snow buds killed by last winter's temperatures that reached several degrees colder than -20F. It blooms for close to 6 weeks, starting with tiny chartreuse flowers that expand into large creamy blossoms over a couple weeks and then remain on the tree for quite a while. Some years the tree has a lot of red fruit that the squirrels eat, but not every year, and they seed around a fair amount. They also hang onto their leaves quite late, which is a mixed blessing; quite ornamental with orange to deep red leaves, but a problem if we get heavy snow in October which bends the still-leafy branches past full recovery to their original shape, though it hasn't broken any branches on my trees. I wouldn't recommend Kousa for a front yard in my zone since this year it looks quite patchy, but for your slightly warmer zone, it should be fine, since we had no bud damage at -17F.

So as Mainegrower said, it's largely personal taste. The trees' overall shapes are different, they both have several seasons of interest, but not in the same way, and they feed different critters. If you are largely going for flower power, you want the Kousa as it blooms for about 3x as long, but I find the pagoda truly charming. Do you have room for both, with one near the property edge and one more central, or the two near opposite sides?