I think Saanens and Alpines are both very good dairy breeds. I don't know if it matters to you, Saanens are (almost) always white, whereas Alpines have a few more colors. I think I would go and look at them, and decide among the ones with a kid at their side based on how their udders look and feel, and partly on temperament too. I hear it's a bit hard to learn to milk (I'll be learning in May), so you want girls who are not going to fight you on the milk stand. It's also just easier to manage goats that are more tame, and it can be a personality thing to some degree as well as a training thing, so do try to get ones that are acceptably tame/friendly.
As far as the long term, what matters here is your plans for the long term. Do you want to get a large herd eventually? You have to breed them (probably every year, but some exceptional individuals can go nearly two years) to get milk - what do you plan to do with the kids? Do you want to eat them, sell them for meat, sell them as dairy animals, sell them for pets (check your area carefully before you decide you'll do this), keep them? This will determine your breeding plans. The reason that this matters now is that it would help you to figure out what you're going to try to sell before you get them. If you're planning to breed them to a meat breed (like Boer or Kiko) and eat them or sell them for meat, then it doesn't matter whether you get two of the same breed. If you're planning to sell them as dairy animals, you might be better off getting two of the same breed so that you can sell purebred goats by only breeding them to one male in any given year (whether they're registered or not, you'll probably get a little bit more from purebreds - of course, you'll get a lot more if they're registered, as long as there's a market). If you're going to sell them as pets (assuming there's a market for that where you are) then I would think Alpines would be better, since they have more colors than the Saanens. Anyway, I'm sure you're getting the idea.
They're all pretty close in age, so their "years of useful life" are pretty similar. I would be a little concerned about why the one 3-year-old Alpine doesn't have a kid - goats almost always breed every year . . . is she in milk? Did they sell the kid? I probably would avoid her unless there was a good explanation for her lack of a kid AND she's still in milk. That's just me, but it's possible that she's not fertile, and then you'll just have a $200 pet goat. Also, depending on where you are, that sounds like a pretty good deal to me, I just got a yearling last summer for $250. As long as they have good milk production, that sounds like a reasonable price (to me) for goats of that age with kids at their sides.
I have a LaMancha (with the little ears) and an Alpine. They're both pretty good as far as temperament goes, and I haven't gotten to milk them yet, so I'm hoping they're both good milkers.
Good luck with your goaty endeavors! They have a lot of personality . . . and are pretty good at escaping if they want to. I hope you have a good fence