What's the difference between N150 and N300 modem routers?

I'm upgrading from an old Speedtouch 580 on an ADSL line because the wiFi signal won't reach the farthest room (possibly due to copper hot water tank in the way?). I tried a Belkin but the connection would drop after a few hours. I know N is better than the old b/g standards but what substantial benefit is there over N150 or N300 (probably Belkin)? Is it range or other features? Total footprint required is probably only 30 metres but there are a few stud walls and wooden floors in the way