For me, choosing what to paint on depends on what size the piece will be. For 36×48 and under I prefer Masonite or hardboard. Masonite has a lot going for it: It’s cheap ($12 – $15 for a 48×96 sheet), readily available, and a stable surface to paint on. Anything larger Masonite becomes too wobbly and unstable, it also gets quite heavy.
Over 36×48, canvas is a better choice; it doesn’t bend, it’s light and looks great for multi-paneled pieces. Pre-stretched canvas can be expensive, so I just buy the canvas and stretch it myself. Cheaper, but a lot of prep required.
For this blog I’m using Masonite.