Do not think it will make much difference either way.
I get similar speeds on mine yet there is about 20m of telephone cable between the master socket and the router. Your call.
Theoretically I suppose the twisted pair ethernet cable would be better shielded … but the telephone cable carries a MUCH higher voltage so would be less likely to suffer signal degradation in the first place.
As SeZo suggests, I seriously doubt if you’d notice any difference over 15 feet … or indeed 30 meters.
Personally I’d go with the telephone extension as they’re cheaper and easier to hide