I've never used a Magellan, so I can't speak about them.
Have always used Garmins and I have four of them at the moment, one of them is the 760 and I like it. All GPS have room for improvement.
The main to not do if you put your large screen GPS into RV or truck mode, is to compare routes with a cell phone GPS. By using the truck / RV mode, the GPS will generally keep you off of the very small roads, sharp cornered roads, roads with excessive grades, away from overpasses with low overhead and roads with weight restrictions. Whereas the cell phone or car GPS units ignore all of that and will give you different directions and drive you batty trying to compare the two routes.
I will stay off of the small residential roads with the 40' DP and toad if the 760 routes us another way. Out in the country I'll ignore a lot of its warnings because I don't mind narrow hilly roads that others may not want to travel. When in those situations you do need to consult the atlas because the RV GPS my route you way out and around certain areas resulting in a lot of extra miles but remember it's doing its job that you set it to do. You can turn off of the RV / truck mode if you want to.
I like the Garmin 760 and recommend it to people. Also, in heavy city traffic I like to turn on the voice instructions so I can watch the traffic and not the GPS - the Garmin has very good speakers that you can hear. Always compare pricing and shipping costs to Amazon and you can read a lot reviews there.
Bill