co.uk has quite a few years over the other, but both will probably continue to do well. Nominet was smart in how they set things up giving co.uk owners a number of years to have first dibs on their .uk version. With that in mind, I'm sure UK numbers will stay fairly high in both variations. As far as paying goes, personally I would probably pay more for the co.uk than the .uk, but only because it's the more popular of the two. To people who live in the UK it may be different..