Great Lakes, 1000 Isles.
Charts would be used up here and down South. Keys, Bahamas, Carribe, etc.
Is there any real difference between GPS and GOLAS?
How difficult is Open CPN to install and use?
looks like the OP is in the UK,and the charts cost a bit more there.
Navionics is probably still a good choice - but if you are getting a tablet.., why not get the tablet version?
GPS satellites are American
The first GLONASS satellites were launched by the USSR, and the system is now operated by Russia.
In the USA receivers that use both GPS and GLONASS are becoming more common, and sometimes give a better fix than those that use just GPS.