I found the MM – the older one – really clunky in use, but it may be easier if you’re used to MM software already. And if your original mapping is transferrable, you’ll save money there too.
I think the Satmap works really well – best of the bunch if you want OS mapping on a GPS – more intuitive, big, clear screen.
Garmin technically does it all too, but the screens are a tad small and OS maps don’t seem to display well compared to other options and some of the menu nesting arrangements are just plain annoying, like the trip computer re-set.
If you have a smartphone, have a look at ViewRanger software as well, which would probably be a winner on value for money, since you don’t have to buy new hardware. It works really well too.
Overall, they’ll all do the job and I suspect you’ll get used to working with any software with experience, a bit like a computer OS, the human brain is pretty adaptable, so it’ll eventually work out how things function, but for me anyway, the Satmap is significantly better than owt else displaying OS mapping, but it’s not the cheapest option out there.
There’s a Lowrance thing out there as well, runs Quo, again doesn’t seem very intuitive in use, like the MM it’s kind of computer software ported to a GPS unit rather than designed from the ground up to work with one like the Satmap or Garmin.