An instructor will point out all the likely "traps" on a route as well as pinpointing potential spots for turnabout and reverse around a corner. I did 3 pre-tests before my test and it was the best money I ever spent as I passed first time.
A map will only show potential routes, but you won't learn anything from a map. Given the waiting times and hassle of doing the test, I'd definitely choose the pre-test option.