Typically you would take Stevens Pass to Leavenworth anyway. That said, Stevens Pass is certainly no picnic. You certainly would want AWD. I would definitely not recommend a midsize unless it had AWD.
WSDOT keeps a page updated with weather info about each pass: http://www.wsdot.com/traffic/passes/stevens/
It's hard to answer right now because both passes are likely to be easier to drive by Friday, but you never know.
My 2 cents: I would cancel. I've driven Snoqualmie in the snow many times, and Stevens a few times, and the conditions this week are the worst I've seen. It may improve by Friday but I would advise anyone without much experience driving the passes to avoid it.