First of all, great choice on the Ontario machetes. The 1095 steel and heat treat of OKC's milspec machetes are much better than the Asian and South American models.
The size just depends on what you plan to do with it. If you plan on doing a lot of ground clearing, get the 18" model. If you plan mainly on chopping, go with the 12" blade.
ETA: BTW, ontario-knife-store.com nearly always has the best prices on all things Ontario.