Depends on the monitor, most monitors are now widescreen (its hard to get one which isn't).
Stick with whichever stays in line with the native ratio, otherwise it will looks bad.
Native ratio of what? Even though we don't have much information, I suppose he wants to use one monitor and can choose between the two. Since graphic cards have no "native resolution" and can output anything up to whatever that specific card he's using, ... I don't get your point.