U/D lock or folding lock?
I think you need to start from the point where you say that any lock is breakable, if the thief is sufficiently determined. But I do think your insurance arrangements will have a bearing on what lock to buy.

So, rather than asking "what lock should I get to prevent my bike being stolen?", I think it makes more sense to ask "what lock do I need to get so that if my bike gets stolen, the insurance co will cough up?"

So essentially rather than asking your question on here, it might be more useful just to contact your insurance company and ask them for a list of approved locks. You don't say where in the world you live, but in the UK there is a "sold secure" scheme which I think a lot of insurers use, with different levels (bronze, silver, gold) required depending on the value of the bike. 

Its also worth noting that the lock on its own is most likely only part of the story - my own policy places restrictions on what I can lock the bike to (must be immovable) and where and for how long I can leave the bike unattended.

On the Sold Secure scheme itself, I've spoken to people who say that it is all a marketing ploy to get you to buy certain locks over others, and their rating of a particular lock has no bearing on how good the lock is. This may or may not be the case, but again I'd stress that its not about preventing the bike from being stolen, its about the insurance company paying out.

I'm making an assumption that your bike is insured. If it isn't, you're really taking a punt when you buy a lock. It might be worth going over to the "Sold Secure" web site (or seeing if there is an equivalent scheme in your country) but as I say it is questionable just how objective these schemes are.