I think that the difference in acceptance between regular and early decision for brown is very difficult to calculate accurately, so your main deciding factor on where to apply early should be where you would most want to go, not where you think you have the best chance of getting in (especially when the choice comes between Brown and Harvard).
I went to a small school in Los Angeles, applied EA to my top choice and got in. Though there were some other factors that influenced my decision, when it came down to it, I applied early to the place I was most interested in.