If you're not in a hurry you can wait and be safe.  As long as you sign up before it expires you can choose from whatever is put up there.

I was debating this very same issue this afternoon.  I have 50+ days left in my step up, currently using a GTX 770.  
From what I've read it seems that reference is mostly issued, though sometimes other versions may become available depending on stock.
I decided against waiting and went ahead to started the step up.  The reference is more than good enough for me, anything extra is just that - extra.
 

i have a gtx770 2gb w/acx and the 970 had an option with an acx cooler but i went for the 980 reference instead i do hope they give an option for 980 w/ acx cooler.