I currently offer packages with a small percentage discount that goes up with the largest package. My clients pay the whole amount up front, so the small discount is justified in my opinion (clients can also choose any number of sessions they would like). This has always worked well for me. I also just wrote a 6 week training/wellness program and may market it in the new year. It would include 2 training sessions per week as an example of a different option.
With the number of clients you have, you also need to look at scheduling options to make sure you can accommodate what clients are looking for. I would think both ways are effective–it comes down to a few things: cost, scheduling and timing of payments are a few things I can think of to consider.