The only places I have seen LP or Propane torches used were automotive junkyards where the goal was a quick and sloppy burn through cut, or to heat up rusted fasteners.
The main advantage is the cheaper price of the gas. The disadvantage is that it is a less focused flame better suited to destruction rather than construction. I would think that the torch tip and torch set would be different, but I haven't really studied a LP torch set-up.
On the farm we use both the oxy-acetylene torch and an electric welder. The electric stick welder is used for serious welding of thick metals where deep penetration is needed to insure a permanent bond stronger than using nuts and bolts. We use the oxy-acetylene torch for cutting and modifying metal and heating rusted fasteners to ease their removal. We also use this set-up for mild welding and filling in cracks or holes by melting in metal from welding rod, or if you are good, baling wire.
If you are wanting to weld structural frame components, roll bars, headers, etc..., go electric. A good chop saw can do a lot of your cutting instead of a torch set. We use a lot of WD-40 to loosen up rusted fasteners.
Keep in mind that an oxy-acetylene set up requires a tank lease that runs around $100 for a year or $300 or more for a lifetime lease if they will sell you one. I have heard stories about guys with multi-year leases not using up their tanks quick enough so that the inspection/certification of their tanks expired before they traded them in for refilled ones. Then the welding gas co. tries to charge the customer for recertification testing or for new tanks.
It would be nice to have a MIG wire feed welder, but we very seldom have a legitimate need for one.
I know of a Shell Valley Cobra locally that had a 110v tiny MIG wire feed welder used to weld major components. This car was built in a rented public storage space. Every time he tripped the breaker for the whole building while welding, he had to wait for someone else to complain so he wouldn't get busted.
When he was sanding and prepping the body, I know he fogged everyone elses stuff due to the wire mesh ceilings and non divided shared rafter areas.
I've never ridden in the car, but he is afraid of the power of his mild 351C engine and never ventures out beyond city streets.
I realize I went beyond answering your questions, but thought our farm welding applications might prove interesting or informative.
SpecialK would be another good source of information.
Fast Farmer