It is truly hard to advise someone on this. The terminal environments here in the FAA are changing. The FAA is seeking to split as many Tower/Radar terminal facilities as fast as they can. They are beginning to trend more towards the Atlanta Large TRACON example. Pretty much the same as a Center, just using 3 miles and running approaches, etc. The Center environment depends on the center. Centers are pretty diversified, but it all depends on the area you are assigned to specialize in. In my center, ZJX for example, my area has sectors that we work aircraft from the ground up to unlimited altitude. We work enroute traffic, climbers/descenders, I vector people for approaches on one end of the sector and put have other aircraft level at cruising altitude on the other side. We also have a small area in one sector that we are permitted to use 3 NM separation as if we were an approach control. The other areas in the center vary on their traffic types... some are just high altitude and enroute, others are like mine and work traffic to the ground.
What you received on the test is really not a big factor. As long as you passed and progress in the training is all that matters. Even in the Academy you really don't get a good handle on things. It all begins to come together when you are actually training on the job with an instructor behind you. The time to certify really depends on the FACILITY not the TYPE of facility. ZJX has one of the fastest training programs in the nation from entry to CPC because the availability of our simulators, the amount of trainers available and our staffing. Other centers like ZMA, ZNY take several years longer due to staffing and delays in their training program. The same happens in TRACON, it all depends on that facilities training department and their staffing levels. If there is no one there to train you, like Miami Center for example, you're going to sit for months without doing anything but office paperwork as support staff.