I keep a pound of butter in the fridge and have never had a bike stolen. It also doubles up as an elephant detector, as they leave footprints behind.
Bods will report that their disk lock is best as, like me, they have not had a bike lifted off the street.
Buy whichever you like, can afford, will match the colour of your bike, hair or whaterver criteria you fancy applying. As St Eptoe (all bow) suggests...
If they really want YOUR bike, they will have it, no matter what.
A disc lock makes it just a tiny bit harder to wheel away, that's all. No need for cutters, thermic lances, hacksaws or snapped off biros, just two reasonably ablebodied blokes and the limited intelligence between them sufficient to lift... and a van to lob it into.
Old freezer vans are popular as the insulation deadens the noise of the useless (but expensive) alram going Wheeeeeeeeeeeee Wheeeeeee all the way to the breaker's yard. A breaker's yard who will then strip the bike and sell the parts on e-Bay where they will be snapped up by Tossers (and tossers) as dealers' prices are a rip-off, mate.