my two cents
let me preface my opinion with the fact that i rarely get up, so if that makes my opinion invalid...meh..to each his own i guess.
i prefer graffiti on a wall or in its natural environment so to say. something about graff on a canvas on a canvas doesnt rub me the right way. Wildstyle burners, a zone piece for example seem out of place on a canvas.
I think the part that throws me off the most is the demensions of a canvas. When i look at a piece on a wall i tend to look at it from different distances up close and far away. I like breaking a piece down letter for letter, foreground to background. Pieces on a canvas tend to seem confined. I tend to just glance at the piece rather than break it down, because a scaled down piece is all there in front of you making it easier to just give it a quick look rather than appreciate it.
Beyond that pieces on a canvas regardless of the background used, simply seem out of place. They look like a closeup cartoon of a person on a blank sheet of paper, completely out of context. i like seeing pieces on a wall or train, because in a way "the scenery" seems to give it a setting and background. in other words graffiti seems out of context when not on a wall train etc.
That being said, certain styles the more abstract styles, lend themselves to canvas easier than others probably because the pieces themselves tend to be more about composition rather than just letters.