Just got my first "real" bike. With it I bought one of the Abus bordo locks - http://www.amazon.com/Lock-Abus-Link-Folding-Black/dp/B00449E7T8 because it seemed pretty compact.
Most of the time my bike will be inside my house or at work, which is secure. But I'm sure I'll want to take it to the store, maybe visit some friends nearby instead of driving. So right now I'm guessing 15% of the time it would be elsewhere besides my apartment/work.
So I want a second lock for those times. I live in a major metropolitan area, and though it doesn't appear on the list of top cities for bike thefts, I wanna do my best to deter theft, and I think I would feel more comfortable with a second lock.
I lived in Europe for a while and the ring locks that lock the back wheel were popular. I had an Axa Defender on my bike, w/ the cable that loops around stuff. Of course this was a really cheap bike that nobody wanted, so it worked fine.
I was thinking about getting an Axa Defender (w/o the cable), or maybe the Abus ring lock. That would take care of the back wheel, and then the other lock I got would loop around the frame and front wheel. Then both locks would be attached to the bike so I wouldn't have to carry them separately, and I'd always have the option of double locking it. Of course it would add a little more heft to the bike. The one down side would be that if the first lock is broken they can carry the bike away.
The other option I was looking at was a mini-u lock, either the Kryptonite or OnGuard. I'd obviously have to carry this with me separately. I don't have any experience w/ u-locks though.
Thoughts, suggestions?