If you are intending to use either of these trees in a drought tolerant situation, I do not believe that they would be happy. Stewartia, in particular, prefers a moist (but well-drained) siting.

I planted a stewardia. It was in full-sun and it did ok for a year. The 2nd year it got some kind of borer that killed it fairly quickly. I really think it was totally stressed out and would have done much better in part-sun and more moisture as rhizo says.
I have three styrax. They are gorgeous in the spring, then they are completely unremarkable. After a tough summer, like this one, they tend to loose some leaves early. They do not color and really don't have any fall interest. One of my trees flowers beautifully. The other two really don't much flower at all.
Carolina Silverbell is more drought tolerant, but doesn't like all day sun. UGA says "# Two-Winged Carolina Silverbell
(Halesia diptera var. magniflora)

* little-known native tree that reaches about 25' tall and about 20' in width
* smaller relative of native Halesia tetraptera, which reaches 30-40' in height
* multi-stem, low, branched tree
* should be used in filtered shade conditions only, not full sun
* beautiful white bell-shaped flowers hang from the branches in early spring
* leaves turn a bold yellow color in the fall

Vitex can be trained into a beautiful small tree. It is very drought tolerant. The bark is silverish colored in the winter.

Waxmyrtle can also be pruned, is evergreen and is drought tollerant and adaptable to any sun situation.

Here's some information from Urban FOrestry South. I didn't open the page, but it lists some 250 trees. http://plone.urbanforestrysouth.org/Resources/Library/select-native-flowering-trees-of-georgia

Hopefully you'll find something that is suitable to your needs!!

GGG

Wow, GGG, you just described the behavior of my styrax perfectly. Mine are on the edge of my woods so they are not in full sun.

For native flowering trees, two of my favorite choices for feature trees are Grancy Greybeard (Fringe tree or Chionanthus virginicus) or Sourwood (Oxydendrum arboreum). Both have eyecatching flowers. The fall foliage on sourwood can be outstanding - I have been enjoying the show for a couple of weeks now from mine. After they turn red they fade to an almost translucent pink before they fall off.

Franklinia alatamaha is another one you might look at. For a change,it's a regional native that even the yankees up north try to get their hands on. Somewhat available in atlanta area tho. Not for a dry site.

Was you sourwood difficult to "take" after transplant?
The reason I ask, is that everything I read says they will be so. I bought one with a 3 gallon root system, but small on top at a native plant sale at Ashville's botanical gardens two years ago. Although it hasn't grown at all, it has lived and the leaves did color beautifully this year. I do occassionally water it in drought, just because I'd hate to loose it.

Do you think some native trees just don't do as well (like styrax or stewardia) in our climate as our climate has changed? Concrete, heat especially holding in night heat, pollution etc.? Sometimes I wonder...

GGG

Well, GGG, I have never planted a sourwood, I have oodles of them growing naturally here. You should take a few of my baby ones to have more! I'm always potting one up to make way for a wider path or to plant something else (they are too good to throw away).

I'm sure that some of the native trees are stressed with some of the conditions in which they are placed - the factors you list and even more. Unfortunately the exotic trees selected for the nursery trade are tough as nails and able to stand the worst of conditions (shopping centers, highway median strips?). Luckily our own native red maple cultivars are doing a fabulous job at shopping centers all over town.

I just brewed my first batch of aerated compost tea and I hope to use it to restore to the soil some of what has been lost over time. Perhaps that will help these guys survive and even thrive.

Esh, I'd love more of your sourwood babies!! I'm going to be digging out a sizeable area of bamboo, and will want to replace it with natives. Many of the pecans here are in bad shape. They will need to be removed in the next few years when we have the finances. That side of the yard will be all native (hopefully), and the sourwoods over top of all the buckeyes I've got germinating in pots would be spectacular in the fall! Blazing yellow buckeye foliage with the red sourwood!!

GGG

I'll second GGG's recommendation of vitex. I'm only on the second season with mine, but it took this year's drought in stride and seems to be doing well. They make good specimen trees and have lovely blooms in late summer when blooms are needed in gardens. I think a vitex would be a lovely focal point for a bed.

i don't grow either of those but second ellen's recommendation for the american fringetree, esp. for a full sun location. the ones that you may encounter growing in the understory do not compare to specimens on a sunny site.
they do not grow too large, can be pruned into a beautiful form with good branch structure and attractive silvery winter silhouette. the spring display can be very showy and is fragrant. they are not unattractive the rest of the year but my tree has no appreciable fall color (a dark yellow/brown).
another medium sized tree that is ablaze with color in my area now is the chinese pistache tree. they cast a lighter shade than some other trees with large, single leaves and appear to perform well even in parking lot islands.

Here is a link that might be useful: pic of mature fringetree - full sun

Pansyface, I'd like to "third" our native fringe tree for your situation. A few of them are lightly fragrant, which is quite noticeable once they get some size on them, and the females bear blue fruit later in the summer that the birds like (if you tuck a male in somewhere for pollination). Just a few weeks ago I got a fragrant female from Specialty Ornamentals in Watkinsville that was tissue-propagated, I think, from a select parent plant. As a bonus, the foliage is larger and handsomer than the other two babies I have and, like them, is still green while leaves fall all around. I was told it gets some fall color but haven't seen that yet. The only negative I know about this lovely tree is that it is a little late to leaf out in spring. Bloom is just after the native dogwoods and often overlapping a bit.