KF8 does have larger line spacing, but I don't find it to be a problem. You might though. Actually, I'm not sure the line spacing is a problem with KF8 or just a problem with the firmware on the Kindles that read KF8. At any rate, mobi is probably your safest bet.

KF8 has a lot of new whiz-bang features. However, unless a book is designed to take advantage of those features you won't see much difference from a book formatted with mobi. Chances are most of your Nook ePubs won't take advantage of that many of the KF8 new features.