I prefer to use our local mart at Huntly, the boys there handle the animals more kindly. I find them a bit coarse at Thainstone by comparison, they shout at the animals and whack the ground with sticks to make them move. I'm constantly told I will get a better price for my stock at Thainstone but a few pence more per kilo doesn't justify the petrol for a small number of animals.
It depends why you want to know really, if you are buying stock Thainstone is a bigger place so perhaps more choice? ...... but you may pay more too if what I'm told is true.
The rare breeds show is very well attended, big and noisy poultry hall so bring ear plugs for the auction, I find I can hear the auctioneer better with the background noise muted a bit. A lot of well heeled enthusiasts often inflate the prices so buy a catalogue and contact the sellers later if you miss the things you wanted, they often have more at home just as good.
Thainstone also has regular outdoor auctions of everything from pallets of building materials to ibc tanks, they are worth travelling up for.