Mike,
In the ideal world you need both.
A monopod for long lens work will give you a couple more stops over handheld in terms of sharpness unless the subject is really moving, but isn’t much help for landscape shots where you need big depth of field and a long exposure to blur water ect.
For that you need a tri-pod.
The best setup I’ve found for fishing shots is a smallish carbon tripod ( mine is a Manrotto ) with a medium to small ball & socket head, so with the legs extended but folded it can be used like a monopod and still do normal tripod work..
It’s a very light setup and I carry it everywhere mounted to the back of my pack.
A good monopod weighs about a pound ?
Carbon weights about 40 or 50 % less than alloy but costs 3 times as much ( here in Oz anyway ).
The new lenses around with image stabilisers are a good thing as well.
The Canon 70-200 F4 IS is one of the sharpest zooms ever made..
www.dsaphoto.com
A picture is thousand words that takes less than a second while a thousand words is a picture that takes a month.