Both will work I suspect about the same.

Ethernet cable can go 100 meters and run gig speed with no loss. Even if you were say to run

modem --100m ethernet --- router -- 100m ethernet --- pc

you would get no loss and gig speed (assuming your equipment can do gig).

The coax cable is slightly different. You are increasing the distance to the ISP. The exact distance limitations depends on many factors but will adding 50ft of cable matter... most likely not. You would have to be at the very limits.

Unlike wireless wired cable that is within the specified distance has 0 loss. If you attempt to go over the distance most the time you get a 100 percent loss.