I love them both for girls. For exactly the nickname possibilities you mentioned. I used to like
Makenna more, but I don't think I'd use it now that it's gaining in popularity. I also like
Mackenzie on a boy.
Spellings: I don't like the random upper case letters.
Mackenzie is my prefered spelling for that one. But in Mackenna, the ck looks weird to me so I prefer
Makenna.
Aging: There's nothing inherently un-age-able about either of them. Compared to some of their misspelled trendy peers, they will age just fine. And I know two Mackenzies (female) in their 30s already.
Combos: I'd pick a shorter middle name than the ones you suggested.
Mackenzie June or
Makenna Bryn, for example. Using
Emily and
Jennifer, what about
Mackenzie EmMackenzie Ember
Makenna JenaeMackenzie JennaMakenna Jenelle
Gavin and Mackenzie/Makenna make a fine sibset.