I've done both, and am happy with both decisions.
I think the determining factor is your goal with the horse.
I was looking for a pleasure mount and ok with knowing I'd be a forever home even if my mare became unfit for riding.
I did sign stating that if I can't keep the mare, for whatever reason, I MUST return her to the rescue. But, it also stated that if I can't keep her I can find a different adopter (approved by the rescue first of course).
Had I been looking for a prospect to use to move up and then move on, or I didn't foresee wanting to keep her if I couldn't ride her, I would have purchased rather than adopt.
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