When it comes to deciding which bag to get there are plenty of factors that come into play. Most importantly is space. If you have a rafter, or a ceiling support capalb of holding 100-150lbs of dead weight I recommend a hanging bag. If there's no space for a hanging bag, get a free standing bag.
When it comes to which bag specifically to get, I did a little research before purchasing mine, and I found that Title has some of the best quality (for the price) Muay Thai bags. Mine was $100 + additional shipping, but might have been a sale, or it was just advertised as one.
When it comes to the size of bag I would recommend 100lbs. While the 80lber is lighter but gives too much per strike for my tastes. And the 150lber is too heavy and risks injury per strike (at least IMO).
Sig courtesy of that photo-matic magician limba
I went out the way that I fight. I went out on my shield. Thatís it. Done. --Chuck Liddell
My goal is to end my opponent --Carlos Condit