Have a look at this KB article: http://support.apple.com/kb/HT5043, i.e. the last section: Notes on working with unified libraries in iPhoto and Aperture
Both Aperture and iPhoto can now read libraries created by the other Application; only some items can not be edited or displayed by the other application, e.g. print products.
The difference between importing and opening will only matter, if you already have an Aperture Library and an iPhoto Library. After importing you will have all images in one place and can search them more easily - you do not have to switch between libraries; and you can create slideshows, books, web pages using images from both libraries, if you unite them.
If you only have one library so far, it does not really matter if you import or not; the only difference will be, that the library afterwards will be an Aperture library, if you import into Aperture. Then double-clicking the Library will open Aperture and not iPhoto.