Well, from an architecture standpoint, green slope and green speed are directly related... how sloped a green CAN BE while being constructed has to do with environment and the length of the grass. I believe this was talked about a few weeks ago in a thread talking about the green speed and wind factor at Pine Valley.
As far as the true factor, i would make the same preference as you. I'd be interested to see other people make judgments about green speed vs. trueness. I would obviously prefer trueness, but in my experience, trueness has almost always been paired with 10-12 stemp speeds. Exceptions coming on courses like RCD, Portrush, Portmarnock, Ballybunion.
All the private courses in the home area are all very, very fast, and very true, though trueness can also be contributed to the amount of play... Courses such as Myopia are a WONDERFUL putting test, because in prime season, the course is rolling at a 12, firm, and very, very true.