Note you can actually import your iPhoto library into Aperture without copying it, and leaving the iPhoto library in place. In fact, the Apple document on moving from iPhoto suggests trying this first to see how things import, if you like it, etc. If you do this it will leave the photos stored in iPhoto, but import all the faces, metadata info, and create the database in Aperture.
I did this when I was considering switching. Once I decided to make the move, I just created a new Aperture library, and then imported my iPhoto and chose to copy the images. Then, deleted the iPhoto library. I think I also had the choice of just relocating the masters into Aperture.