Pencil lead is more popular on the rivers simply because it's cheaper and can be "customized" to suit the fishing situation. Also, you need to consider how each type is fished, it's quite difficult to bottom-bounce with split shot, and pencil lead is cheaper than bouncing betties.
Split shot are used a lot in lakes under floats more-so than pencil lead is. Split shot can be more effective than pencil-lead when float fishing, simply because it balances the weight out accross all the line instead of a big chunk of lead in one spot.
Doing a direct weight comparison between pencil lead and split shot can be difficult because there are different diametres of lead available, and also different sizes of split shot. If you are using it to float fish, just add and remove split shots as neccessary to achieve the right float balance.