All I know is, 3 years ago I was doing this same research. I wanted a unit for OFF ROAD, and Boating, biking and hiking. After it was all said and done It was Garmin hands down for ease of use and software availability. Most fishermen use Lowrance. So it was a hard step.
Afterwards I was happy I went with Garmin. I can transfer SD cards from one unit to another (Garimin 60 CS to my Garmin 188C) smoothly. I can add extreme detail to the hand held with the USB link and as long as I have my laptop, I can change the detail anytime, but it hold enough to go 650 mile trip easy. I seldom re program it. However, the new Garmin that replaces my Handheld takes SD cards! Whoooo that would be the sheet! But to expensive to update just for that as I have no problems now. I carry the handheld 100% of the time, for last 3 years I have dropped it, soaked it, left it in the sun on dash, I mean I have abused this sucker like Consumer Reports would only dream. It's Garmin all the way for me!
And a side note, at same time my Father in law Bought a Lowrance, the software was so expensive (Double Garmin's) he wouldn't buy It, so my dumb ass (I was trying to avoid the expense that's why I went Garmin) bought it for him. His unit is very hard to operate and navigate the screens as compared to my garmin, where my father of 73 can work it no problem.
Stop in a store that sells GPS's look at the accessory rack, Garmin!