I've never played Guild wars, but for £20 would you buy Guild Wars 2 or get Metro Last Light. I know they are two different games, MMO and such, but if you was spending money what one would you get?.
Guild Wars 2. Metro is bound to drop in price in a matter of weeks, whilst GW2 will provide you a magnitude or two more hours of entertainment. That's assuming you'd like both games, of course.