It would be absolutely better to get a mare rather than a stallion. You'd have to have an exceptionally well behaved stallion...I've known one, but I've known a lot more mini stallions that are a LOT more trouble than a full sized stallion. They are fully capable of hurting you pretty badly. Too many people treat them like lapdogs rather than like livestock. Minis can be tough to have for that very reason; you will likely also have more trouble getting appropriate vet/farrier/dental care for him, minis can be difficult to work on so there are a lot of people out there who refuse to work on them, and there is a high rate of dwarfism out there. When you take all this and blend it into an animal that is likely to have even more attitude than a full sized stallion, and the size to squeeze through places you've never even imagined he could, you're better off passing it up. If you must get a mini, do yourself a favor and get a mare or gelding...this stud will still want to breed your mare despite his size, and she is likely to object to having a mini trying to mount her hind legs or bite her on her belly!
Also take into account whether your mare has ever even seen a mini before...some full sized horses *really* dislike minis and ponies. I've seen a mini stud kicked unconscious by a bigger horse who hated him.
