I am no expert on these matters, so please take my comments on face value. However...
I have tried both and I have found the wireless signal strength and coverage to be MILES better on the RT-N66U. I mean like no comparison. I had an old Netgear DGND3300 originally and struggled to get 5GHz N upstairs in the house, and my Samsung Galaxy S2 (which has a pretty rubbish wifi antenna) could barely connect downstairs.
I found the EA4500 to be hugely disappointing, and not much better than the DGND3300.
The RT-N6600 is in a different league. I get full signal on 2.4GHz and 5GHz all over the house! I cannot understand why it is so much better, but it is. I reckon half the street could connect to my wifi now... which is hardly an advantage, I know, but the signal is so strong everywhere it's bordering on ridiculous.
I am completely delighted with it, I have to say./