Having used both routers myself.
The only difference is a SLIGHTLY stronger wifi signal from the 2504n.
Other than this the 2504n is capable of faster wifi speeds.
However both routers can easily handle a broadband connection of over 30 mbps... (had these speeds via home networking)
So for a upto 20 mbps sky connection, both are FINE.
Don't worry and honestly you won't notice the difference when browsing websites.
Both routers are rock solid stable. You can plug them in and literally forget totally about them. They never crash etc.