All wood gives the same heat by weight when seasoned properly. Hardwood burns longer because it is more dense, softwood more quickly because it is less dense (and before any book read smart alec chimes in, yes, . balsa is a hardwood, I know, I am using the terms used by people who sell firewood).
So volume for volume (e.g by the builders bag) hardwood gives more heat but usually costs more. You are unlikely to be given a choice of tree, but ash and oak are great.
Best choice is to get a wagon, chainsaws and training and go cut up a load of trees and season it yourself
Second best choice is to buy in lorry loads of random trees from tree surgeons and the forestry commission and process them yourself.
The most economic way to run a multfuel burner if you have to pay for seasoned wood is to buy smokeless fuel by the tonne.
If you must buy builders bags of logs, shop around for a local bloke who delivers. £50-£60 for a builders bag (nearly 1m3) is normal. The price for ash wood on that logs for sale site above is beyond stupid. You could buy half a tonne of smokeless for the price of a crate of logs. Thats just daft.