Vanilla goes through patches, naturally there is a limited amount of content, enough for around 2-3 years before people start having everything.
Kronos 1 is complete and most has been farmed while Kronos 3 is quite new with the next patch around a month from now being BWL.
I suggest Kronos 3 unless you want to play where all the content has been released and naxxramas been farmed for a very long time thus all have full or nearly full naxx gear.
of course kronos will launch TBC in the future but that is still far away.