P16 - the decision largely revolves around what a/c you fly. I used to use a tri-fold when I was learning in C172 / C150, but then found that when I started in a PA28, a tri-fold would get caught under the control wheel when doing full / free checks. I thus abandoned using the tri-fold, but found that an A5 clipboard that strapped on my leg was the next best thing. I now fly a Fuji FA200, which will catch event the A5 board, so I tend not to use a strapped-on knee board these days. Once in the cruise, I just keep the board in the a/c pocket by my left calf and if on a long transit, would tend to strap it on.
For those who have not seen the Aeronautical Chart Folding Method, have a look at the attached link. I've not personally used it, as I tend to fold according to my Local Flying Area, but keep promising to do this one day and try it!
Aeronautical Chart Folding Method