I just want to add my personal experience here and sorry if I'm not clear enough as I'm not English.
BTW : I've started using iPhoto and then switched to Aperture and imported the full library into Aperture. It is true you can do it, but it is not as smooth as expected. I had (and still have : I backup everything on at least 2 different HD) some 5.000 pics and most of them were edited in iPhoto. One of the biggest difference between the two programs is that when you edit a photo in iPhoto (no matter how deep is the edit) it creates a second file eaxctly the same size of the original and if you further edit the already edited photo it creates a 3rd file. Assuming you have a 1,1Mb Jpeg file, once you edit it you get to have 2x1,1Mb Jpeg files (unless you reduce the size of the original). Those files are stored in different areas (one called "Originals" and one called "Modified") of the iPhoto Library. Aperture works different : when you edit a photo it just create a few Kb file storing only the info required by Aperture to keep track of the changes you've made: so no duplicates are created. So Aperture is much more efficient in managing you disk space.
Now the big trouble : when you import the iPhoto library into Aperture you import ALL the files : both the originals and the modified ones and, therefore, your brand new Aperture library will show you all the duplicates and there is no automated way to fix it, unless you, as I did, go photo by photo and delete the one you don't want to keep. I've tried all the possible workaround to avoid this manual work (forum, discussions, Genius bar, One to One), but no way.
One more : the iPhoto hyerarchy is respected in Aperture, but because iPhoto uses "Events", while Aperture call them "Projects" (which at the end are small libraries) when you import the library in Aperture all the iPhoto events will be under a different folder called "iPhoto library" and to use all the Aperture potential you're obliged to move every single iPhoto event to a an upper level.
I don't want to discourage you, but Aperture is a much better software and you can do much more, so I would suggest you go directly with Aperture with no doubt, also because it makes no sense you first get used to manage efficiently iPhoto and then you have to re-adapt your "style" to Aperture.
One last comment on referenced images : if you use referenced images (which reside outside the Aperture library), you can't benefit of the Backup functionality Aperture provides you (called "Vault"), and you have to manually backup your files on an external HD.
BTW I'm now an happy Aperture user, but I had some bad days in the past !
Hope this helps.
Ciao.