My experience is some of the 757's have the worst leg room in the industry. I'm not sure if it's a model, or a carrier's configuration choice, but I have been "pinned in" in the back of UA and DL 757's. I can't recall the back of a NWA 757, but would expect them to be similar.

I'd book the 737 flight on Alaska. It's coming into fog season in SEA. Last year this time I was booked on a SWA flight that was delayed (inbound SWA flt could not land in SEA due to fog). Alaska was running strong in and out of Seattle. My company re-booked me on the AS and I was gone!^

Alaska's equipment seems to be some of the newest in the industry. So do you want an aircraft that's a few years old, or a few decades old? Your choice.