Basically with silver you don't get the part hot enough to glow, and with brass you do. Neither will destroy the steel, depending on what it is. It is more a case of discolouring the flux and getting impurities in the joint. You can burn the steel by overheating it without the protection of flux or blanketing gas. When you hit the right heat, your flux is good, your rod runs easily and you don't have a lot of junk in the filler, and you didn't get any more heat in there than you know you need to. The joint looks good.
On propane:
- Gas is nicer it burns cleaner as far as not getting a lot of whispy crap all over the shop, doesn't blow if you drop something on the hoses, or some idiot cranks the pressure. As far as the gasses are concerned in a home environment, Propane tanks out of doors are everywhere, and oxygen is not flamable, so it might help an insurance case a fraction. It is a little cooler which probably doesn't help much for welding, but should be fine for brazing. Might be less poisonous (?).
- If you already have some propane like a BBQ, you can just handle that out of the same bottle. Maybe the gas co's bottle is safer (?), anyway if you use a ton as with heaters you probably want the larger bottle. I got rid of my BBQ, and intend to use that bottle, and a regulator I have for a forge, so I am saving a little money. I still need to have Oxygen though, at least for the torch. You could probably do lugs in a propane harth set-up without oxy.
- The propane is a little cleaner in the joint. I really don't have the comparison of all the gasses to go by, but some folks swear by the propane/Oxy set-up. If accet is working for you, no reason to change, mostly.
As far as welding is concerned, all the above advantages apply, except the Tinman said "life is too short to weld with OXY propane". That didn't bother me too much because I do have TIG, and I'm not in a rush. I do believe they may have made some progress though. They brought out a whole range of tips for propane, and I inquired as to whether these new tips were hot enough, apparently they are fine for welding. I doubt they work as well as accet from the heat perspective, but apparently they work well enough to catalog them. I haven't tested any yet, so I don't know for sure. I will be buying them though.
I use 4130 mostly, and simple chromo, and don't know about other steels. I do know some people are doing very different steels with propane.