It depends what you want. Personally I would rather buy a youngster; I can see what I am getting (or what I might be at least), plus all the costs associated with foaling are paid by someone else! There's no need to keep and maintain a broodmare, and then what happens to her afterwards? I have just bought a weanling and the reason I did was that I didn't want to end up with two more horses, I only wanted one, and I'm not happy about the idea of selling on the mare afterwards. There are too many "broodmares" passed from pillar to post. I wanted to bring a horse on from scratch as it were, so I know his background and his mileage, and I can bring him on as I want to.
I bred from my mare and the only request was that I didn't want a black colt...guess what I got! Hes also going to end up at least a hand bigger than I wanted.
On the plus side I love him dearly, I have no one but myself to blame if hes useless and its been great fun. It has also cost me a fortune, I could have bought 2 youngsters for what he owes me.