Yahtzee where to 6’s near end of game
Assuming that it is the second last round, you can expect about 1 point more by going with 6s. 

In case you go for yahtzee you get 50 points now and expect about 16.3 points including a bonus for 4 sixes or more in the last round for a total of 66.3 points.
In case you go for 6s you get 65 points now and expect about 2.4 points for yahtzee in the last round.

If 3 sixes were enough to get the bonus you should go for yahtzee instead.