In the past, the West End had higher ridership than the Sea Beach, but according to the 2010 ridership statistic, both lines have similar ridership, with the West End a little bit higher. This may be simply because the West End has more stations, however. The Sea Beach's ridership is increasing in a faster rate.
Bay Parkway is the only station on the West End that has more than 2 millions riders in 2010, while the Sea Beach has 3, they are 8th Ave, Fort Hamilton Parkway, and Bay Parkway. That is why I said that extending to the Sea Beach actually benefits more people. Unfortunately, it isn't going to work. Both lines don't need a second service anyway.