A lone ace or a lone king — which do you keep?
Keep both when you have both: A♥ K♦ is worth 0.4783 per coin. Forced to choose one, the king wins — 0.4635 vs 0.4579 on 9/6 Jacks or Better.
The exact numbers — A♥ K♦ 4♣ 7♠ 9♥
EVs are exact — every possible draw enumerated, not simulated — at max bet on 9/6 Jacks or Better. Check it yourself: the analyzer runs the same solver in your browser.
See all 32 holds of this hand ranked →Why
Hold both: two high cards give twice as many ways to pair up for the guaranteed 1-coin return, and that is worth more than the extra draw card. The tiebreak between them surprises people. An ace makes exactly one straight, A-K-Q-J-10. A king makes two, that one and K-Q-J-10-9. That is the whole difference, and here the king wins even though the 9♥ being discarded is one of the four nines its second straight needs.
Chart line #15 of 18, '2 unsuited high cards', covers this hand — keep both. The chart's line #17 ('one high card') doesn't rank aces against kings, but the 36-line Double Bonus chart does the same thing explicitly: 'K-Q or K-J unsuited' is line #30, a lone ace line #31. See the full solver-optimized strategy chart, or drill hands with exact-EV feedback until plays like this are automatic.