Most firewood suppliers only sell one of the two, so their answer is decided before you ask. We sell both. Here is how they actually differ, what each costs, and which is right for how you burn.
It all comes down to moisture, not marketing
Freshly felled wood is roughly half water by weight. Burn it and most of the energy goes into boiling that water off instead of heating your room. That is why wet logs hiss, smoke, blacken the glass and disappoint. Everything else in this argument is a footnote to that one fact.
Kiln dried means the wood has been through a controlled drying process and comes out reliably under 20% moisture. Every log in the load is dry, not just the ones that happened to sit on top of the stack.
Seasoned means it has been cut, split, stacked and air dried. In a British climate, done properly and given long enough, that gets you to somewhere around 25%. Done badly, it gets you to whatever the weather allowed.
The number that matters
Under 20% is dry enough to carry the Ready to Burn mark. Between 20% and 25% is a good fire if you handle it well. Above 25% and you are heating the air outside your chimney.
What kiln dried actually gets you
Consistency, mostly. That is worth more than people expect.
• It lights first time. No coaxing, no second firelighter, no opening the door to poke at it
• It gets a cold stove up to temperature quickly, which matters on a February evening when the room is cold and you want heat in twenty minutes rather than an hour
• Noticeably less smoke, so less soot on the glass and less tar in the flue
• Less creosote build-up, which is the sticky residue that causes chimney fires
• It can carry the Ready to Burn mark, because that certification requires 20% or under. Ours are WS7474/00001 for logs and WS7474/00002 for kindling
• It stores badly less badly. Dry wood left somewhere imperfect deteriorates more slowly than seasoned wood does
The cost of all that is about £45 a cubic metre against our seasoned hardwood, which is roughly a 20% premium.