Just what is says on the tin. I don’t really know what the use is of having both in the game, since as far as I know they do the same exact thing.
The difference is that torch can be used to cut metal wall afaik. I found some elevator in basement of DenseUrban building, planning to cut its doors as it seams to lead only downwards. Dunno if there is something there, metal doors are surrounded by metal walls, which makes me suspicious
Acetylene is hand held, smaller, using gas welding. An electric welder is big heavy and takes up a lot of volume.
You haul torches into labs etc to cut open areas you need too.
The main difference is that the welder uses electricity. You can put a UPS or storage battery mod on it and power it from your car, giving it infinite use with solar panels.
The Acetylene torch uses welder tanks, which are limited and non-renewable. They’re more efficient charge-wise so you can use one a lot longer than a charged welder, but once they run out they’re done. Unless you find more welder tanks, which are quite rare.
Hmm, so you can cut door with a welder too? Have to try that.
Actually I don’t think you can. That’s probably the best thing to save acetylene torches for, assuming you have some solar panels to charge a welder.
I have personally never know torches for being small and I distinctly remember them being pretty dang heavy. However they do indeed cut through metals doors and can act as an alternative to a forge in I don’t know how many recipes but those are things the welder is not.