Personally, I think it really comes down to 2 main things - choice and convenience. I've been to a lot of art galleries around the world, and I've seen some amazing prints - but I would never buy one and try take it on a plane or train with me. As much as I love art, when you're out exploring the world, you don't want to be lugging around a large canvas.
When you look at choice, there is so much more variety online than any art gallery you could visit. Even if you're looking for a specific artist, odds are you could bid for that piece online. Hands down, I believe it's best to buy canvas art online in most circumstances. It's convenient, there's more variety, and you can spend as much time as you like deciding on if that particular print is right for you!