I vote for leasing. Cost where I'm at is about $200 per ten year lease or $20 per year for the pair of oxy/act. bottles.
The argon bottle on my mig runs as i recall $150 on the same 10 year lease.
The other thing with a lease is, bottles have to be inspected and tested periodicilly, which is a responsibility of the company you lease them from.
If you own the bottle(s) and one fails inspection or testing, you'll need to purchase another new one, whereas if its leased it's company responsibility to replace it with a known good bottle.
The other upside is owning them you have to wait on having them filled as opposed to trading the empty for a full one when leased.