I think 1.e4 ends up having more theory because of the Sicilian defense.
As an aside, if you go to http://database.chessbase.com, you'll find that 34% of the responses to 1. e4 is 1...c5, and 30% of the responses to 1. e4 is 1...e5 so you can make a guess as what to study first based on opening frequency.
For years I have been reading that 1.e4 is too demanding on lower level players due to theory. So one should stick with .1d4 or others to lower the workload. But now some masters told me lately I should definetly try 1.e4, as it is hard to develop tactic skills and learn the power of the pieces without having that fase of open games.