How much Ethernet cable do you think you'd need? I have my modem downstairs and run 25ft of Cat7 cable upstairs to my router and it works well. As long as you won't need more than Ethernet degrades I'd go with Ethernet, especially since as Johnny pointed out that the signal is borderline to out of spec.
I should add that it's been connected for over a week to one of the outlets with the -18 dB signal. Also, FWIW this is the outlet that I connected to when the tech connected the cable service and this was the line he tested with his testing device. He didn't offer more than a shrug regarding signal quality.
Ethernet run would be essentially from one side of the house to other, then from second floor to basement. I'm thinking 75-100 feet.