From what little I know about GPS, I think one of the main features that's not discussed to often is the unit having an electronic compass. Reason being, at slow speeds or sitting still sure, the GPS knows where you are but it has no idea which way your facing. A unit with the electronic compass always knows which way your facing in relation to your waypoint so it can better point you in the right direction. From what I've been reading the problem with the SI units is the chip update speed so this might not help with that but then again, maybe it would. Thanks.
Mark