Unfortunately you probably will not have a choice in which one to do ... trust me, I need one and just had my 4th ankle surgery in 4 years and can't get a replacement due to my age, and I am 41.
Since ankle technology right now only lasts 10 years and can only be done twice, they won't even consider doing a replacement until at least age 50 .. so I have to hope my last surgery gives me another 9 years and that ankle technology improves in that time. Ironic they can make a 30 year knee and hip but not an ankle and that I am TOO young to get out of pain and stop having surgeries.
Sorry, had to hop in here because this is all too familiar a topic for me. You might want to check out the foot forum here, many more conversations regarding fusions, etc.
Good luck, I feel your pain ... literally!
Oh, just noticed you are in Scotland .. things may be different over there .. here in the US they will not do it for someone your age HOWEVER I do know from what I've read that elsewhere (Switzerland for example) will do replacements at a younger age. Sorry I didn't notice your location sooner ...