Finished Attic or Central Air? What's your opinion? Thanks!

[Chris S.]

Hello, my current home is a 2 unit, that was converted in to a 1 unit.  I am planning on moving out soon and I was considering converting it back to a 2 unit (it should cash flow at a much better rate).  

If we had to choose between central air and a finished attic (which would probably be considered a (walk through) 4th bedroom on unit 1, which is currently a 3br. (unit 2 is a really nice 1br), which would you choose and why? 

Also, I realize the central air would have to be split so that could be a problem, unless we make it only for the bigger unit.. so what would you do if it was a 1 unit vs a 2 unit.  Thanks for any input!