1. The most important thing about buying a new
bike is to make sure it fits. The only way you'll know if the bike is right
for you is to size up the bike and make sure that the bike's geometry matches
your body's geometry. Ask questions and do some research.
2. If possible, try to find a shop that will
let you demo the bike on real dirt. Five minutes in a parking lot won't cut
it. You wouldn't buy a car without a real world test drive, and a bike should
be no different.
3. Don't belive the hype. Just because
your favorite rider or best friend rides a certain bike, that doesn't mean
that's the best one for you. Have an open mind and be realistic about your
needs and ability.
Depends. Air shocks are more tuneable to a riders weight/riding style and are typically more progressive (spring rate ramps up towards the end). There are some linear air shocks. Air shocks are typically lighter as well. The nice thing about a coil is that even if you blow it out, you can still ride it out. Each has it's pros and cons. Not neccesarily better or worse.
Following trying both types of shock I'd say the spring shock is better for really hitting the large bumps and not worrying about whether or not you will be able to make it off the trail if you have mechanical problems also, it is more of a set it and forget it type of shock...
An air shock is much more tuneable and generally lighter, I haven't had any problem with any of my air shocks leaking however, I know over time they will develop leaks and I'll have to check the air pressure more and more...
...You might not be able to tomorrow
that's like asking if Shimano or Sram is better. There are true believers on either side of the fence and they will never agree. You'll see a lot of well-thought out arguments for both if you look around (you'll also see a lot of poorly thought out arguments, and a lot of repitition of BS from the LBS).