Personally I think JOSM is easier to use than Potlatch.
It means I can actually do stuff without having to memorise dozens of keyboard shortcuts, I can just choose it from a menu or toolbar, with custom presets if I want.
And the command stack is very useful in that it shows what I've changed, and can undo it all if necessary - in Potlatch its possible to accidentally move things and not notice.
There is the option to "Update data", which will check to see if anyone else has edited it, and warn of conflicts and allow them to be fixed. You could get edit conflicts with Potlatch if you are using it in "Edit with save" mode.
And as my surveying usually involves taking photos of just about everything, its very handy to load all of the geotagged photos in JOSM, to show where things are.
Plus it doesn't use Flash, so is less likely to eat up lots of memory and crash, crashing my browser as well...
As for Merkaartor, its just strange...