Hmmm, the problem of buying by volume.
There’s one fallacy above that i’d have to comment on – “softwood rubbish”. Not really, chemically you get near-as-dammit the same energy output per unit mass in either type, for a standard moisture content. Hardwoods are just denser, generally, so greater output per m3. That said, if you’re taking the extremes, balsa is a hardwood…
And like you say, the standard ‘volume’ measure is also a bit cobblers, since logs don’t exactly tessalate perfectly (we have a woodfuel seller around here who seems to think the Hilux is an SI unit…) So your metre cubed load looks pretty variable. If it’s a metre cubed of freshly felled wet hardwood it’ll weigh a metaphorical tonne and burn like a not-very-burny-at-all thing. If it’s a metre cubed of seasoned softwood, you’ve a lower potential output in total, but at least it won’t burn like a fresh turd and gum up your flue.
Best bet is to look for someone who sells by the tonne at a guaranteed moisture content – 25-35% is the norm for stoves etc. Ignore anyone who tries to sell an “oven dry tonne”, it’s a mythical construct of 0% MC, which is practically impossible to achieve and maintain. Kiln-dried also isn’t really worth the premium, you can air dry timber to 25% easily enough without extra energy input.
(Sorry, rant over. Used to consult on these things…)