Back on the original topic (sorry): Arguably the "within you" translation is better than "within your midst". The Greek is ἐντὸς ὑμῶν (entos hymôn). Entos
is normally an adverb, but it's functioning as a preposition here. Hymôn
is "you" (pl.) in the genitive case.
All the standard Greek lexicons I looked at, with one exception, give the meaning as "within". Here's an example from Mounce's Greek dictionary:
improper prep., gen., inside, within, Lk. 17:21
The one exception is the UBS lexicon, and I think they're reading this passage back into it.
Now if you want to argue that "within" a plural "you" is the same as "in the midst of," go ahead, but realize that you're doing some exegeting along with your translating.