Hanging bags are great because without clean alignment, technique and speed, you won't generate any power -- and the bag will let you know.

An "average" height person of about 80kg will do well with a 60-lb or 70-lb bag, but you can go as low as 40-lb if you're smaller. Anything lighter will swing way too much. WaveMaster-style standing bags have several problems: (1) you can't throw low kicks, (2) they're usually way too light and the padding is too thin, and (3) you'll be kicking through to the central plastic column in no time. Once you start kicking the plastic core, you'll either hurt yourself or snap the column off.

Just get a bag that's the right weight for you. No swing (too heavy), and your joints won't hold up. (Talking about joints, wear hand wraps, please!) Too much swing (too light), and it's just ridiculous -- not enough resistance to give you good feedback on your technique, and you end up chasing the bag around in an unrealistic manner.

Heavy leather is the best material -- it'll last the longest, stay the cleanest, and be less abrasive to your skin. But they also cost more than canvas, so it's really up to your budget and preference. To save shipping costs, definitely get an empty bag and fill it yourself per the manufacturer's instructions. Especially for low kicks, make sure you fill it correctly so it doesn't turn into a rock at the bottom.

Long banana bags are good for low kicks, but taller normal hanging bags are just fine, too. Canvas will tear up your shins. Fortunately, canvas banana bags are rare. If you get a banana bag, make sure to get thick leather, as the longer bags tend to stretch more, and pretty soon the bottom will be dragging on the ground.

I didn't mention speed bags. It used to be rare to find them in Southeast Asian kickboxing gyms. They're more of a Western boxing toy. Given a choice between the two, a hanging bag will give you more bang for the buck than a speed bag. :)

This post turned out to be more of a buying guide, but you can use the same criteria if you're simply trying to choose the best bag to use in your gym.

For workouts, you might look up some Muay Thai routines. Generally speaking, 2-minute rounds mixing punches and low kicks, with 1-minute breathers, make a good workout.

Have fun on the bag!