Buying works best for my needs.
I looked into the adoption process at several local rescues but the fine print was a deal-breaker. I understand that these organizations are doing what they feel is in the best interest of the horse, and I support their attempts. But their stipulations didn't mesh with MY needs, and I suspect it will prevent some other worthy people from taking the adoption plunge, too.
The main issue was that I would not be able to sell the horse, only return it to the rescue. Considering I was looking at young horses to start my way, their potential re-sale value was a consideration in my prospect selection. Not to mention that sometimes a horse just doesn't work out for some reason, and if I couldn't sell it, I wouldn't have the cash on hand to buy/adopt another horse.
Also along those lines, they could reclaim the horse from me at any time for any reason. While I guarantee any horse in my care will be appropriately cared for, that contract clause would leave them open to taking away said horse after I'd put in all the training, potentially allowing them to sell/rehome the horse at a bigger profit. Not a chance I was comfortable with.