Punching the speed bag will definitely be lower impact on your joints and knuckles, and keeping the arms raised and moving is a great way to get tone. That being the case, I would probably recommend speed bag for your needs. However, like you said, a heavy bag offers a much wider variety of activities and and may make your workouts more diverse and interesting.

Hey Eric - you'll probably want something in the 100lb range. If you want the bag to swing for you, you could get a little less and work on footwork. If you just want it as a tool for striking then something a little heavier like 100 would be best.

I would even go higher and reccomend a 150 lb bag. I like to be able to stand and crank shots as hard as possble without the bag flying all over, and if you want to "vent frustration" i would assume that is the way you'd like to do it as well!

Hi Elizabeth - the 70lb-90lb range would probably be most appropriate for your son. Leather is generally considered the most durable exterior, but since he is new and will be using it as a stress management tool rather than a boxing training device, any exterior should be fine (including vinyl and canvas).

If you want more sway and a more rewarding feel when just starting out, go lighter - say 60lb. Or get the Hydro Strike back, fill it only partially with water and top it off when he's stronger and wants to hit a heavier bag.

This bag is actually filled, but if you did get an unfilled bag here's what to do:
Put some sand (25lb should be enough) in a heavy duty garbage bag and put at the bottom of the bag, this will keep the bag still. Then pack the rest with a soft stuffing. This can be old clothing, rags, shredded newspaper, pillow stuffing - you get the idea. If you can get some carpet padding line the bag with it before putting in the stuffing. This will give the bag a more uniform look and feel. Make sure to pack the stuffing down tightly.