1. Shielded Metal Arc welding (stick), with out a doubt over brazing.
2. SMAW is faster than Tig, not always the case with mig. Some advantages are penetration, ability to weld outside in windy conditions. Tig is preferred for aluminum, stainless, titanium, and exotic alloys. It can be used for mild steel but is slow compared to SMAW.
3. That's just too broad of a question. In theory and practice the weld bead should always be stronger than the base metal. With stainless it is extremely difficult to make code or x ray quality welds with a flux covered electrode. Impossible with titanium. Different processes for different applications and metals.
4. Alternating current actually switches polarity back and forth. Direct current does not. DC+ is when your machine is set up to run on DC and the lead connected to the stinger is hooked to the positive side of the machine.
5. I'm a professional welder, not a metallurgical chemist. For more info about base metals I'd check out www.wikipedia.com. For more in depth answers specific to welding look at Lincoln's web site.
6.No. Brazing is more like soldering. You use a filler material that has a lower melting point than the base metals. Think of using a hot glue gun. Gas welding actually melts into the base metal and the filler, if any is used, is intermixed. Gas welding produces a very sound weld of x ray quality and can be done in any position. It's just slow compared to the methods that came after it.