I've been using prompt for a day now, and so far it feels better than the others. The app design feels really clean, the method of expanding the keyboard to handle modifier keys and frequently-used non-alpha keys works well without grabbing too much screen real estate (this is clearly visible in the screen shots for what it's worth).
Finally, the terminal emulation has been flawless for me so far. Emacs runs well (and is quite usable with Prompt's modifier key placement, unlike in other iOS ssh clients I'd tried; remember, folks, ESC is Meta, Meta is ESC). touchTerm had some screen lag/partial refresh issues for me with Emacs -- these may have been fixed in later versions, though, as I gave up at some point.
In short, I like it.
obDisclaimer: I don't know the developers. I don't have a dog in this fight. I do like the app. I'm not the only one who likes it though -- see the daringfireball take here: