For downhilling or jumping I would definitely say get a full face. If you're into pedaling, then get something else because you don't want heat stroke and it's easier to drink water without something in front of your mouth. Kali has some cool non-fullface helmets.

When I downhill, I ride with a Fox Rampage that I got off REI's website for around $100. You can spend way more but I don't know what the benefits are. Fox is a well established brand so I trust them to keep me safe. I don't need a fancy TLD helmet so I'm not going to pay for it. When I'm just riding around, I use a skate helmet because I already have one.

You probably already know this but helmets are only good for one impact. What I've learned from a friend is, "You'll feel it when you hit your head, so that's how you know when your helmet took its shot"

A lot of people downhill in motocross helmets. I just read a report on pinkbike where people who make helmets were saying those helmets are a lot stiffer, they're designed to break at speeds you would hit on the freeway. You're not going to be going that fast on a DH bike so you want a helmet designed for lower energy impacts. I would say steer away from MX helmets