this is not something i have ever thought about much as the care of these guys is essentially the same. But from the top of my head:
i have found western painted to be the hardiest captives.
esaterns seem the most prone to shell problems but are the easiest to "tame" (though all painteds are pretty docile and responsive)
southerns attain the smallest adult size.
westerns attain the greatest adult size.
in my opinion midlands are the least attractively colored.
the southerns have the smallest range and while not a threatened (sub) species they are the least plentiful in the wild i think.
southerns tend to cost more beacuse of the red strip and its popularity.
westerns are the only ones i would characterize as aggresive feeders.
the eastern is the only north american turtle to have aligned carapace scutes.
some westerns and some easterns are very beautiful turtles indeed.
i am not at all certain about statistics but it seems that every year there are many more eastern and southern painted hatchlings offered for sale. So maybe westerns and midleands are bred in as great numbers.
many many westerns that i see in stores are obviously wild caught.
check this out:
[url=http://www.chelonia.org/Articles/diffpaintedturtles.htm] painted turtle article /url]
and this:
more painted facts and maps