Slow! I don't care!
Two liter and portability.
What you do, is get the cheapest bottled water you can. It's usually (considerably) cheaper than the soda, by weight. Get whatever has the most bottles for the lowest price. Then you either drink the water or pour it out (possibly into another container for later drinking!) and fill the bottles back up with soda from the two liter bottles.
Portability solved, and it's still costing you less per ounce. I haven't actually found a place just selling bottles, but if I do and that's even cheaper...
At the very least, never, ever, buy the free standing 20oz bottles. Unless things are massively different wherever you are, those overpriced pieces of crap cost as much as the two liter ones... sometimes more. Refrigeration is apparently enough to triple the cost per ounce

The canned stuff is almost as bad. Vending machines in general are ridiculously overpriced (Though I've seen
some exceptions, on individual stuff.) and should only be used in absolute emergencies.