I have grown Stuttgarter Giant, planted in spring, and they performed very well. Downside for me is that they are "flatter" than globe onions, and peeling them (or rather chopping off the root plate) made them awkward in the kitchen but don't let me put you off considering them. The onions stored very well over winter so I imagine the sets would too but bear in mind that the sets are smaller in size than a full size onion. Garden suppliers tend to have much better controlled facilities to store their sets and bulbs, so it could be a hit and miss - you may be alright, but if stored incorrectly, they might try to throw up shoots before it is time to plant out.
With regards to planting now, I really don't know but I'm almost certain that it is a spring planted onion which surprises me that they are available (are you sure you are not ordering them now and they'll post them after the new year?) - a part of me feels that any planted now, come spring, they would gear up to throwing out a seed head which will shorten their storage capability in the 2nd year.
Maybe it's worth an experiment - if you do decide to get the sets now, maybe plant a few and keep the rest until spring and see what happens?