What Garmin call their big strength - seemingly unlimited number of models - can be very confusing at times.
I would say that the 126 and the 128 are the same unit with the only difference being that the 126 has a built-in antenna while the 128 has an external. I wouldn't be surprised if they had the same "engine" as Garmin's older handheld receivers, i.e. 12, II+ etc. The fixed receivers have a benefit of much larger displays and keys, which is very much welcome feature in a fast RIB.
I own a GPSMAP 180 and I cannot recommend it. The update rate (when zooming or changing map coverage) is way too slow. Maybe it can be tolerated (although I doubt it) when doing 5 knots, but at 35 the things can get really scary!
Add to that low display resolution and the fact that the continued support for G-charts is questionable now that Garmin have started using Transas charts and it all gets even worse.
I had an opportunity to test the latest Garmin chart-plotters based on the Transas chart (or BlueChart as Garmin is calling it). And these units are really great. Transas charts are miles better that the old G-charts based on Navionics technology. All symbols look as they should and no longer do you see black squares and wonder what do they represent - a lighthouse, a cardinal mark, a beacon or what.
My favourite plotter was GPSMAP 2006C, with a 256-colours, 6.4-inch display with a resolution of 640x480 pixels! But it is also very expensive - approximately US$2000. I don't even want to know what the big brother - 2010C - costs.
I also tested the successor to the GPSMAP 180, the GPSMAP 182. While the unit looks just like the GPSMAP 180, that's where the likeness ends. The 182 features an upgraded 10-level grey, 360x240 pixels display (as opposed to 4-level grey, 160x240 in the 180). The update rate is around 1 s. The 182 is also WAAS/EGNOS compatible.
Given that the 182 costs some US$600 (which is more-less the same as the 180 costs here in Europe) and that the Transas charts are far superior to G-charts and that the Transas charts are much cheaper it's not difficult to conclude: if you want a chart-plotter in this price range go for the 182. If you can afford the 2006C or 2010C don't hesitate to check them out, but I would imagine that there are some other interesting alternatives in this price range.