As long as you store the seed somewhere cool and dry I think you'd be better placing your order asap so as not to miss out on anything...Usually the new list cancels the old one, so don't think you can order from the 2013 list.
That being said, I haven't ordered from Mesa in many years as I find getting stuff from them (and the US in general) is a bit more difficult/expensive than Europe as I think you may need a Phyto certificate. You may also require CITES cert's for CITES1 stuff which I think Aztekium is..
The issue sowing in the next couple of months is keeping the heat down, if you have a propagator etc it is easier to sow in cooler weather (late autumn/winter) to get the headstart as you have greater control of the environment.
That being said, it seems I'm just about always sowing, and just put down another lot this past weekend. Have bought way too much seed over the past years and needed to get some in before it goes off. But still I'm thinking of placing another order soon...it's an addiction I tell ya !
Yikes, Pseudolithos Mccoyi at 1.20 a seed ? Must be very rare over there or something. I must be sitting on a gold mine, my plants self set viable seed all the time. Just sowed about 50-60 of them on Sunday and there's 4 horns ripening on plants at present. While not exactly common, it's not quite in the same class as cubiformis or miguirtinus.
Good luck with your sowings !