^There is nothing exclusive here. Nothing prevent you from getting both. By the way, insurance won't prevent a thief from stealing your bike and there is no guarantee that you will see the color of money from your insurance company.
Regarding the bike alarms there are plenty of choices and some of them are so cheap that even if it's not very efficient you don't lose much

Here is a 4$ one
http://www.amazon.com/Bicycle-Securi...rm+system+bike

A few more
http://www.amazon.com/Bicycle-Anti-t...rm+system+bike

http://www.amazon.com/QQ-Tech-Creati...rm+system+bike

http://www.amazon.com/Generic-Motorb...rm+system+bike

A padlock alarm is better than a simple alarm since in the former case the thief who is not afraid by the alarm system can simply throw away or break the alarm

There are also motorcycle alarm disk locks that are way stronger even stronger than most Dlocks depending.
On a bicycle they can be placed on the chainring, the disk brakes, (spokes: not advised)

http://www.amazon.com/Trimax-Alarm-D...larm+disk+lock