Some tank bags are too high and you might find that you can't reach the bars without the bag getting in your way when the bag full. Be sure to try the bag on the tank before buying or using. I have old tank bags that attach to my bikes with straps. Newer tank bags have magnetic thingys and will fly off in a crash.
I'm not sure about the U.K. but here in the states we just can't have enough storage. I say get both, your gonna need 'em in time anyway ? My old pan still has room for a few odds and ends, so as they say in the Castro, strap one on.