RTW:

I'm going to try and expand on Warin's answer above. In the first sentence of your post, you wrote: "Not much of an idea about GPS...", so, I'll try to cover things starting from first principles. Please don't interpret this as me talking down to you.

A "GPS device" consists of a number of components that provide different capabilities or functions. Not all devices have all possible components. So, let's look at what these are:

1) The satellite reception system. This enables the device to determine where you are. Not all systems update at the same speed. A smartphone might only update the position once per second. A dedicated GPS navigator typically updates 5 times The difference between update rates is not important to a pedestrian or hiker, but is very significant to a person navigating a vehicle that might be moving at 90 feet per second (60 MPH, 100 km/h).

2) Cartography. By this I mean whatever maps exist within the unit to display your position on. The Garmin eTrex 20 and 30 that you mentioned come loaded with a 'basemap', which is only a very low detail map. You would need to buy additional maps (either topographic or road, depending on whether your interest is on-road or off-road) and load those maps into the device before it would be of much use to you.

3) A navigation engine (a processor that calculates and shows you routes). This is a hardware component. Like anything else in the world, a processor that is fully dedicated to GPS navigation will be more efficient than a processor that is handling navigation as a sideline task (for example, a smartphone processor). Be aware that there is a rough correlation between the price of the device and the power of the processor.

4) Navigation software. (the program that the processor uses to generate the route, operate the device, etc.) Small hand-held devices such as the Garmin eTrex line are designed for pedestrian use (hiking, geocaching, etc.) They are not designed to support vehicle navigation, although some will generate routes on roads if the appropriate cartography has been loaded.

5) A display. That might sound obvious, but there are HUGE differences between displays. Consider, for example, the difference between the 1.4" x 1.7", 176 x 220 pixel display on the eTrex and the 4.25" x 2.55", 800 x 480 pixel display on the Zumo 590. The little display is fine if you are on foot and can pause to look at it or zoom in or out, pan around, then keep walking. For vehicle use, you generally need a physically larger display that shows you everything you need to know in one quick glance.

Before you can make a wise decision about what kind of GPS device you should purchase, you will need to define what you intend to use it for. If you plan to use the device for on-road navigation, then an eTrex would be a poor choice for you. For the same price as the device and the additional maps needed, you could purchase a device that is specifically designed to support road navigation (including big screen, pre-installed maps with free future updates, stuff like that).

If you intend to spend most of your time off-road but in a fairly small area (say, within an hour or so riding time from your home), the eTrex might do the job for you, but keep in mind it is optimized for pedestrian / hiker use, not motorcycle use.

If you intend to travel long distances (to other countries, for example) and spend time off-road on the way or once you arrive (considerable on-road use is implied by that, otherwise, how would you get your bike there?), then you should think about buying an automotive or motorcycle specific GPS that has sufficient memory to hold both road maps (which support automatic route generation) and topographic maps with high levels of detail (that means more than just the basemap).

I hope this information helps you with your shopping.

As for the question you asked about compass behaviour, ANY GPS will work as a pseudo-compass once you start moving. It does this just by figuring out what direction you are going in, then orienting the map (or a compass rose) to show you how the direction you are moving in correlates with north.

If you want a GPS device that has a true, stand-alone compass in it (the flux valve that Warin referred to), then you will probably have to buy a device that is primarily designed for hikers. Such a device is less than optimal for motorcycle riders. Personally, I think the 'pseudo-compass' inherent in all GPS navigators designed for cars or motos is more than sufficient, even for riding way off-road. All you have to do is move forward (in any direction) for about 5 feet, and the device will know which way you are moving, then orient the map display correctly... hence you will know which way you are pointed.

Michael