I bought seeds for Giant Winter Spinach to try this year. I just have to find the time to prep a bed and sow them. I'm waiting for delivery of floating covers though.
I'm pretty sure I already have some Winter Bloomsdale? Hmm... it might have been Bloomsdale Longstanding. If I have Winter Bloomsdsle, I try them both.
Fedco Seeds - Item: Giant Winter
2555GW Giant Winter Spinach (45 days) Open-pollinated. Selected for its cold hardiness. Recommended for late fall greenhouse crops, or overwintering under mulch. The most productive for late fall/early winter production in high tunnels with up to six times the leaf matter of competing varieties. Not as good springback recovery from hard winters as Winter Bloomsdale. Large medium-green semi-savoyed leaves.
On the same page towards the bottom, they have a pretty thorough germination temperature info for spinach.