Really depends on the site's audience.
I have a bunch of .co.uk's that are active businesses (UK focused) and although I'd buy the .com's if they were available (even for a few hundred quid) I think that my visitors would type the .co.uk first if told the name buy a friend.
Many countries have got behind their cctld, (.co.uk, .de, .co.kr, etc.)
As for one being more valuable in monetary terms - for the domain only sure you might think the .com (more potential buyers and global market) but if your product, business or service isn't global (but UK focused) then the .co.uk is just as valuable as the .com if not more so in my opinion.