When I played Troon I played the Old course first and then the Portland the same day. I found the Portland to be a lot of fun, there are some interesting holes there. Some are weak, but the openers on the Old are pretty weak as well -- its just that it finishes so very well people forgive that.
Portland has more variety than the Old, but its a lot shorter so is hurt more by technology. It also doesn't have the same up/down routing that puts you into the teeth of the wind for the entire back nine, which would be challenging enough with no wind, but is positively brutal when it is howling. The Portland has its strongest hole first, and gets easier from there, the exact opposite of Old Troon.