So I'm thinking of using metal instead of wood for my frame. Angle iron for the legs and support, as well as for the sides and ends. But would it be better to use aluminum or steel? Or am I wrong to use metal and should use wood instead?
It makes no real difference, it's all in the construction methods and which you feel the most comfortable with. In the end, done well, the wood will have a more elegant appearance. But fine cabinetry work isn't for everyone.
Jim
you can use steel and put wood trim to hide it.
A good place to save money could be a scrap yard where people bring their scrap for money. Some decent stuff can be had there. They'll charge some markup from their scrap prices, but should be 1/2 or less than street value.
When I made mine I used 1 1/2" square tubing and welded it.Then a few years later when I took them out I re-purposed the tubing for racking in my shop.
If the beds are designed to fold or move, then metal may be better. If fixed, then aesthetics may be a consideration. I'd think that as far as comfort, a frame is a frame. The supporting material within the frame is going to have a noticeable difference.
Metal takes less room and stronger