Stock difference is 500MHz
4770k running 3.5GHz stock and 3.9 GHz turbo boost
and
4790k should run 4.0 GHz stock and 4.4 GHz turbo boost.
Official release date is not out yet
. Estimates are june/july. Most optimistic are early june and pesimistic push it to mid-late july/august. But it should not be so far as they release 4790 not so long ago and new motherboard with intel 9 chipset are rolling out right now too.
Other question is how will be 4790k priced.
But I'm in same situation I was going to buy new PC with 4790 or 4770k/4771 but now I'm waiting for 4790k.
By reports and rumors and marketing info it should have better termal efficiency and possible higher OC but those are still just rumors.