I prefer Imogen. By using Imagen you would be changing the way it is pronounced from (im-o-gen) to (im-uh-gen). So if you like the name Imogen I would stay with that name. Also, technically if your using the middle name Ayries your child will still have an A in their name. So all your children would still have A's in there names. If that was a concern for you. But I think Imogen is a very pretty name :)