I agree with Shooby, half-day at IOA, full day at Universal. DH & I are always able to do IOA in a half-day, although I don't do coasters, but DH does.
I think it also depends on the time of year you go. My mother and I went down on December 16 last year, arrived at Universal by 9:30 or so, did everything we wanted (no shows), had a long lunch at Finnegan's, shopping at HRC, and about four rides at IOA, shopping at a few places in City on the way out, and we were checking into our hotel at around 4:30.
As for theming, both parks are truly fantastic, IMO. I love the way the "worlds" are laid out at IOA and the music in each one. The Port of Entry is most certainly a very magical spot for our family. Universal feels very much like a movie set to me and very comfortable. I just can't help but smile as I go from street to street. I've never felt rushed at Universal (probably because we dedicate a whole day there, lol).
Hope y'all have a blast!