aviation o2 is cheap anyway - so not much of a saving going for welding o2,
the only problem with the breathing grade stuff is that BOC is sometimes a pain and doesn't want to supply when they know that you want to transfill.
Thats why i get my sruff through work - plus now it is tax-deductable
.
Helium - well big price difference obviously - however dil consumption is low, so shouldn't be a biggy either