I've used all three options (Melbourne Long Term, United and Jetport)..
Answering the original question.. on our last trip starting the Friday before school holidays, we found "G" to be the best position. Very few people on the bus arriving, as not many people coming to Melbourne, but by the time we got to "D", we were full and passed all of the final 4 stops. The only problem was the cost. For the weekend, was the same as for the full week. Not good at all.
We've done United, which is good when it comes to picking up the bus on return, but not so good when it comes to meeting their "3 minute" advertising. Also, not much different to Long Term Parking in cost.
We've done Jetport, which is good when it comes to cost, but atrocious when it comes to being picked up on return. You have to phone a mobile number, that you are given on a little piece of paper when you depart, and after x weeks away you've most probably lost it. None of the information people know about the company, so finding the mobile number at the airport is extremely difficult. Finally, their pickup point is not anywhere near the most popular long-term terminal.. International.. so you have to drag your bags to where they do pick up.
Bottom line.. you get what you pay for.. Jetport is cheap and simple, with some warts. United is middle-of-the-road. Long Term is the easiest to deal with, but the worst for the "short term-long term parking".