If it is a quality of life issue (i.e. the horse is arthritic and generally failing slowly) rather that a very sick horse who is dying anyway, I would opt for having it shot.
The reason being, as horses are such large animals and have a slow heart rate and respiratory rate it can sometimes take a while for the anaesthetic drugs used to euthanise them to circulate and do their job. This can distress the animal as it struggles to breathe etc whilst feeling dis-oriented.
Although much less pleasing for us humans, having the hose shot can be done whilst it has its head in a lovely bucket of carrots and is literally instant. Much kinder to the horse and there is no anxiety or distress for them.
If you feel the time is right, I would act swiftly. A friend diliberated for a week or two to over what to do with her old horse and after she had finally made her mind up, the day the vet was due, she found the horse collaped and covered in blood after geting cast in his stable. The vet thought he had suffered a stroke during the night.
I feel for you and your horse. It is a very sad decision to make but it is the kindest thing you can do for an old horse, particularly with cold wet weather on it's way.
Love and hugs to you both.