1- I chose Raven. Mainly the difference between the factions are the look of the weapons, soldiers, and location of their defend map. The layout of their map could have certain advantages and disadvantages for either side playing on that map as well.
2- As others have said, you have to get to level 10 first.
3- Not really sure. I haven't been buying a lot of new games at full price as of late (last full price game was Killzone 2 back when it launched). However, it's a rental at least.