I'm a total slut.
I will paint on anything. If you browse through the Terminal Distraction shop you'll see that. And people frequently ask me what the difference is, especially when they're commissioning a piece.
(If you're looking for info on canvas panels, sort of a hybrid of the two, check out this post where I talk about them in depth)
First of all, I paint on both because one isn't any better than the other. They're just different and they each have their advantages and disadvantages.
Some Obvious Differences...
The most noticeable difference is that panels are rigid, whereas canvas has some give to it. Because of this I prefer panels if I plan on doing any collage work. I can use as much pressure as I need to get rid of air bubbles without worrying about warping the surface.
Texture is another obvious difference. Canvas usually has a fabric-like texture, while panels can vary. Some are very heavily textured and others are sanded smooth. The very smooth ones have a tendency to show more brushstrokes, which can be good or bad depending on what you're going for.
...and the Less Obvious Ones
Panels are cheaper. It takes significantly less skill to glue some wood together than it does to stretch a canvas on bars.
A piece done on canvas is going to be a LOT lighter than one of the same size on a panel. That means easier to transport, easier to hang. Trust me, wrestling a 5'x3.5' wood panel into the back of my Kia was not fun.
The fabric of canvas does deteriorate more rapidly than a wood panel. It's more susceptible to time and the elements, which is why it's crucial to take care of them properly. If you want more information on how to do that I've got a free guide to caring for acrylic paintings (on canvas or panel) that I'll send you when you sign up for my email list.