This is a broad answer, but it should help.
Prebans also have the advantage of either having/being able to accept a folding stock.
7.62 is the original caliber, but 5.45 is the current caliber. You would have to assume that armies that use AK's switched for a good reason.
If you are looking for top quality, you have 3 choices (postban):
The Vepr which has non-standard looks but is available in a few different calibers. It has a reinforced receiver which is always a good feature but it adds a little weight. It is also made in Russia which adds a little bit of appeal.
Arsenal makes a top quality AK with a milled receiver (a good bit heavier) but is only available (for now) in 7.62.
I would be berated till the end of time if I didn't also mention the undisputed best
bang for the buck. The SAR series, available at
Aim Surplus and many other places, is the classic, traditional AK. The quality of those rifles is a bit lacking but they can be customized to your liking.
Their is also the choice of having a gunsmith assemble a whole rifle out of a parts kit, but that is a different monster altogether.