Personal experience can play a big part. I've only been down the south oxford once and the wind howled and the rain poured the whole way. It's made me not want to return.
All my memories of the Llangollen are of glorious sunshine, gorgeous landscapes, amazing historic structures, loads of wildlife. Lazy, balmy evenings barbecuing on the towpath or relaxing in a drunken haze in beer gardens. Then Llangollen itself is a real prize at the end, whereas Oxford the town is nice but the moorings are far less so.
What could anyone have against the Llangollen? I know it gets a bit busy, but there's a good reason for that!