Since i have both, i'll advise you to get the k3. The screen is way better than the 600. Conversion is not a problem with calibre so far after i switched from 600 to k3, so i don't think you'll find it hard either.
The thing to note is if you download kindle app on your iphone, it will only sync the book with your k3 if you bought it from Amazon. If you don't plan to read books on both devices at the same time then i guess it's not a problem.