Fishing is terrible for terrible so don't even bother with it. Mining is much better, I have 99 fishing and mining and definitely mining makes more money. Fishing is horrible for money.
Mining in the long run hands down if you are going to at some point do forgiveness of a chaos dwarf (keldagrim switching runite), but even if you aren't going to do that, going from 80-99 mining still makes like 30-40mish if you do it in the living rock caverns (not sure what the exact amt is at the moment).
Mining & Fishing would both give you the same amount of cash if you went to the Living Rock Caverns from the required levels.
Overall, Mining would give more cash if you decide to mine Runite Ore from 85-99. It takes a VERY LONG time though, but you'll make over 1B cash I believe.