Do I have a better chance of getting accepted if I apply early action or early decision?
As I mentioned in another response, your odds of acceptance (statistically speaking) can go up dramatically through applying ED. With a school like Johns Hopkins, for instance, you're looking at an overall acceptance rate of around 18 percent. The ED acceptance rate is over double that. So, by the numbers, applying ED offers far better odds than otherwise.
However, this brings up a question: Is the pool of applicants better in ED? In other words, does the strength of the other applicants outweigh the statistical advantage of applying ED? Even further, is this the cause of high ED admit rates?
My perspective on this (and, frankly, I haven't come across enough data disaggregated by the schools to offer a crystal clear picture) is the following: At top 10 schools and right thereabouts, I've found that students applying ED will be generally very strong. Once you get outside this group, the students applying ED will be applying under the plan looking for "an edge."
Why? Because few students will apply ED to a school for which they're a "lock." Most students are applying ED to schools a level or so above where they would usually get in. To simplify, students who're looking at Carnegie Mellon as a target would be inclined to apply to, say, Washington U. ED. This being the case, the CMU ED class (in my opinion) will often be slightly weaker than the RD pool; students are using this opportunity to shoot a bit higher.
However, one can't make the same argument for the ED pool at, say, Columbia or Penn. There are almost no students applying to these schools that feel "comfortable" with their chances for admission to the schools.
So (again in my opinion), the ED pools (and the REA pools, for that matter) of the blue chip schools are rarely depleted by an "applicant flight" to more competitive institutions. There just aren't more competitive schools out there!
In a nutshell, then, I suggest this: Identify a level or so above where you would normally get in. Ensure you've taken the tests in time for the 11/1 or 11/15 deadline. Ensure that your six-semester transcript is solid. If these tasks have been done, use ED to leverage your chances.