Brands:
I suspect they may be all the same... but Cuprinol and I think Ronseal are good companies, a bit more expensive and I think claim to have a 5-year life. On mine it easily lasts that except on the top rails exposed to sun, hail, birds and cats, but if you want to keep it smart you could run a brush along the top without needing to do the whole fence so often.
there are always cheaper own-brand ones in the DIY sheds which will be OK, may not be quite as good.
the Spray and Roller versions are more expensive but as far as I know no better. Spraying tends to drift onto your plants, your neighbours washin, the house and car etc.
They are very easy to apply with a big soft brush and do not need rubbing down or anything (but mud, moss etc must be removd so you can apply it to the wood). Apply the first coat more carefully, brushing it well into cracks and joints.
You can get by with one coat but I prefer to apply two or three. It dries so fast that by the time you have got down to the end of the fence the first end is ready again. It will stain clothes but washes off smooth skin easily and contains no spirits or solvents. Wear gloves or it will get under your fingernails and need to be scrubbed out.
edit
p.s. see
http://www.cuprinol.co.uk/products/ducksback_overview.html
p.p.s If there is a heavy rainstorm before it has fully dried, it will all be washed off. that has happened to me