The only theft I've ever had any experience of first hand was a GPS left on a bike at the Ceuta border. It fits the idea that you'll get opportunist thieves who'll take a GPS unit or camera rather than the sort you get in London who come equipped with angle grinders and plumbers nitrogen. Simple locks and a bag you always take with is my solution.
Try and get into a routine. I always put my GPS in the grab bag I remove the disc lock from. I follow this routine at any stop where I'll be out of sight. The chap who lost his GPS initially thought he'd put it in a pannier, then left it in the passport hut. He didn't have a routine so the fact he'd left it in it's holder or on the seat was a bit of a poor way to end a trip.
Alarms I'm happier with, immobilisers I hate. They are the single biggest cause of bike problems in the UK. If you want a simple mercury switch that makes the horn blow the worst it'll ever do is flatten your battery. Electronics hacked into the main loom with phials of acid that melt the whole bike if they are tampered with and little key fob remotes that are powered by virgins blood I'd avoid.