I personally don't like either name, I just find them really masculine (and the nicknames you mentioned sound masculine to me too - particularly
Mack and Mackie). I don't think it's the "son of" thing that bothers me, or even the trendiness, just the fact that they are surnames and I don't like surnames as first names really.
I think the reason that you can't picture a 30 year old
Mackenzie is because almost all Mackenzies out there are very young at the moment - there are no 30 year old ones yet. In 25 years time, when all the little Mackenzies have grown up, it will be easy to imagine an adult with the name.
I wouldn't put the random capital letter in the middle (
MacKenzie) - that makes it look even more surnamey.