I've had both in 28' boxes.  Different trailers, so the comparison isn't entirely accurate, but FWIW, the 15k provides noticeably more cooling...not enough that the 13.5 would be a deal breaker, but I'd certainly take 15k if I could get it.  Between the two, the 13.5 could generally maintain a 20-21 degree differential to the outside temp with the typical use in the trailer.  The 15 maintains a 25 degree differential.  The 15k difference was enough that I ordered the trailer just to get it (instead of taking an otherwise identical one off the lot).