I've worn both. I owned an HJC flip-up. I took good care of it, always kept it in a helmet bag, always set it on the ground rather than the bike (never had a helmet fall off the ground). One day, while stored in a fleece lined helmet bag, it fell off of the shelf in the garage about 5' to the floor. The chin guard cracked. I sent it into HJC, and they did not seem surprised. They offered charge me to repair the helmet. I bought a Shoei flip up instead.
The Shoei cost more than the HJC, but it is obviously better made, much stronger with a better latch. It fits better and is quieter, though not as quiet as a full face. But I wear ear plugs, so noise is not much of a factor.
I trust the Shoei with my life. I can's say the same for the HJC.
Why do I wear a flip up?
1) It's easier to get on and off with glasses.
2) On hot days at stop lights, I sometimes open it.
3) Store clerks might worry when you come into a store wearing a full face helmet - they think you might be using it as a mask. When I'm just running into a store for a minute I just flip it up.
4) Finally, I sometimes listen to my iPod on long rides. I've modified head sets with ear plugs, and I find I'm a lot less likely to pull the ear plugs out as I put on the helmet.
