Hi Evo,
Firstly Welcome to the ODK Forums.
17 or 20, well that question really rely's on the size and profile of your lawn area. Is it just a smaller flat rectangle or a substantially larger area with no curves etc.
Once you've explained that I'll be able to give you some advice.
Also are these machines the early Solid Decks or the later Series II twin railer versions of the Model 45 ?
Cheers,
BB.