Agreed with the above, while it's not a bad idea to have a disc lock, you need a proper "wired up" alarm system.
Have been using the Scorpio SR-i900R systems myself for all 3 bikes I've owned, and they've generally paid for themselves over time. Mostly just preventing idiots from sitting on/playing with the bike (though some still do it anyway and ask "what's that weird screeching sound?"... no joke..) which gives me peace of mind. Scorpio also has factory harness kits for many bikes you can purchase, to avoid having to splice into wires on the bike, which is great.
A disc lock will do little to nothing to prevent experienced thieves from lifting the bike up and into the back of a truck/van, and for that matter neither will an active alarm system in the right circumstances, but it's at least better than nothing at all.
The most commonly installed motorcycle alarms are the Scorpio and Gorilla systems, have heard of a Talon brand as well.