Bike specific GPS or car GPS with cover and mount
 

Let me preface this post by saying I have never owned a GPS. I am looking at Garmin because they seem to be the most common so I assume have the most maps etc.

The trip I am planning is through Northern Asia, Russia and Mongolia.

Is it worth spending big money on a Zumo model or would a Nuvi (2555LMT or 2597LMT or similar) be just as effective with a good cover. My bike has a 12v cig lighter socket so i'm not too worried about battery life.

I guess the other option is a 62/78 series but the screens look very small and hard to use.

I believe that an SD card slot is essential for adding more maps.

Thanks.

If you were to go by the sticky threads in this forum then certainly Garmin products are widely used.
One key reason for this is the accessibility of Open Street Maps (OSM) which are free and compatible with Garmins.

Personally, I have experimented with a Nuvi which most of the time while I am riding is switched off and out of sight - I tend to get it out at the end of the day to find a specific location, or other times I don't bother and just find a place by old fashioned means - I mention this only because it means that I am not beholden to the GPS technology.

I recommend that you take a look through the last few posts in here about:-
1. Specific GPS manufacturers Vs smartphones
2. Cheap/best value GPS products (other than Garmin et al), such as "peaklife"

Such reading will provide you with a range of views about the utility of navigating by other than map and compass.

ps to me, waypoints are like stamps; I don't collect either of them.