I do think that I'd probably favor the effective top tube length when sizing. Both "top tube" numbers are the effective top tube lengths, right (to avoid an apples-to-oranges comparison between lengths of sloping top tubes)? You can always have more seat tube sticking out of the Coda's frame to get your saddle-to-crank measurement where you like it (in order to have the reach you desire), but you likely can't very easily shorten the reach on the 23" Coda. You could put a shorter stem on it, but I don't think you could get it THAT MUCH shorter (you're talking a difference of 60mm between the 21" and 23" frames).
After seeing the numbers, I think I'd agree that the 21" is going to be the closest to what you're used to. You'll have more exposed seat tube on a 21" (compared with on a 23"), but that's perfectly fine, as long as it's not pulled out past the minimum insertion mark. Your reach measurement is in the same ballpark, so I would think you SHOULD be able to get it dialed in pretty close.
Of course, online/sight-unseen purchases like this are always risky, as you never can know for sure.