Further to my earlier post, I think I once read that the standard Navigator is good for 100-150 metres range, while the Plus can manage 300 metres or more. Of course the range of either is degraded by solid objects (walls etc), so if your layout is designed in such a way that you don't always have a clear line of sight to wherever your Dimax station is situated (eg: if the layout goes round the side of the house, or something like that, and you want to be able to follow the train round there) then the extra radio "oomph" of the Navigator Plus might be a worthwhile investment.
Jon.