Yep, its the same stuff. You'll hear about 'medical grade' and 'food grade', but unless you are buying little CO2 bulbs for pellet guns (which have some sort of lubricant in them too), its all pure CO2.
some people buy a used steel tank and learn the hard way that it was mis-treated and got rusty inside, which taints the flavor of the beer...but that's very rare.
I've ONLY used welding gas in my beer systems, and in aquariums. NO problems.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10