Hi parm, I've used both these units, and have the Raymarine 425 on my RIB.
It's a no-brainer!
The Garmin 126 only displays Lat&Long, SOG (speed over the ground), COG (course over the ground), CTS (course to steer) and heading in text mode, as well as GPS compass display in graphics mode. The old unit I used was quite difficult to read - I don't know if newer units are more easy to view.
The Raymarine 425 can display all of the above in graphics mode (ie extra-large characters) and has the base map (although that's not very useful). If you spend a little more, you can fit a Navionics Classic chart - each of my large chart chips cost nearly £200, but I think that the small chart chips are about £100. Check this out with a supplier though (like JGtech or Sea Chest Nautical bookshop), as stocks are disappearing.
Bear in mind that you would need to fit the separate antenna for the Raymarine unit as it's not integral to the unit like the Garmin 126. The Garmin 126 is also much easier to input waypoints - this is really a chore on the Raymarine unit.
Have a look at Navionics site here: http://www.navionics.com/Charts.asp?...&ChartSizeID=1
IIRC, I purchased my chart chips from them direct.