Light Infantry are smaller in unit size so they will die faster than Line Infantry will. You can also spread your Line Infantry out to 2 ranks deep to get a lot of the muskets firing. Line infantry also have access to ring bayonets fairly early, which makes them a better CC option.

Light infantry are good, but I wouldn't rely on them as a substitute for Line Infantry.

Late period, there is no question, Line Infantry > Light Infantry smply because of rank fire.