Straight to the web can mean many things. What size are you talking? Full 1080 streaming, or youtube compression, or somewhere in the middle?
When choosing a format, you should take into account the story/content first, followed by your expected output format.
While a youtube movie is roughly half (?) the size of broadcast SD, shooting in HD or higher will still yield a better overall image quality than if you were to shoot in SD.
When choosing between SD and HD you also have to take into account the shape of the frame. Is your movie more suited to a square frame, or is it disposed more to the wider frame of HD?
If you're going to Youtube, keep in mind that you may have to letterbox a widescreen video to fit into the standard youtube size window, which decreases your overall image area. That can become a serious problem at the compression/quality levels of youtube and its competitors, especially if your movie has a lot of subtle, fine detail in its visuals.
