You can't say that EBA is easier than Ouendan, because it's subjective once again. Specifically, it will depend on which one you played first. I was dissappointed initially with how easy Ouendan was(all the way through to Ready Steady Go on Normal without failing a song, and even that one only took two tries), but that was because I had already gotten the highest ranking in EBA when I played it. It took me two months to get the highest EBA ranking, yet because of that I was able to do the same on Ouendan in under two weeks(I had every song in the game completed within one week).
Now that I've finished them both, I still think EBA is harder(except for the spinners, which are just too hard in Ouendan), based on the fact that I have more S ranks in Ouendan. Also I have S Ranked Ready Steady Go on Hard, while I still can't even finish Jumpin' Jack Flash on Sweatin' without at least missing one beat somewhere in that mess.
I expect this to become a major issue later in the year when Ouendan 2 is released, and too many people claim it's too easy, when it really isn't.
There are no real reasons for choosing Ouendan or EBA, only personal preferences. I like them both myself. The Agents are better characters IMO, and I like the Agents dancing more than the Cheer Squad's cheering, but I prefer the songs in Ouendan. As I said, I think EBA is a little harder, but then again I kind of like how the patterns seem to flow better in Ouendan at times. I hate the Ouendan spinners, and don't particularly like the EBA ones either. I could go on and on, but the point remains that these are just minor details, and both games are among the best the DS has to offer.