[Winstonm]

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This is plainly worse from the point of view of accurate slam bidding , and I doubt that it gains significantly on game hands. In fact, I'm not even sure that a hand suitable for your 3NT bid actually exists. Can you give us an example, together with a responding hand that would pass it?

Obviously, the key issue to passing is how the 3N is defined.

Qx, KQ10, KQ98, KJ10x or xx, AQ10, KQJx, KJ10x

AKJxxx, xx, xx, AQx or K10xxxx, KJ, Axx, Ax

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This is plainly worse from the point of view of accurate slam bidding ,

I don't think it makes a lot of difference to good slam bidding either way.

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and I doubt that it gains significantly on game hands

Me, too.

Now I am at a loss. We seem to both agree that 4S should be an option - which is what the poster asked. You missed the point I made at the start - that 3N cannot be passable as it was being used. My suggestion to use 3N as passable does not mean it is the best contract - 4S would be right most of the time - but it is showing a particular type of holding outside spades that a balanced (6322) spade hand might decide to play - although it would be rare.

[whereagles]

1NT 2♥
2♠ 3♠

Now:
4♠ = ♠xx ♥KQJx ♦KJx ♣KQx
3NT = ♠Jxx ♥KQxx ♦AJx ♣KJx

I.e.
4♠ = bad fit, bad controls.
3NT = min hand, average on fit and controls.