[pilun]

This dull textbook hand came up the other day.
Scoring was matchpoints, which is important.

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A few choices: pass, 1♠, 1NT, 2♥.

Clearly 2♠ is best at matchpoints.

If ♠Q is onside, might be +140 vs +110 in 2♥, +90/+120 in 1NT, depending on the club break.
One less for all those if ♠Q is offside.

Some will always rebid 1♠, even with 4-3-3-3.
I once played that 1♠ promised 4+♦.
Or you could play that 1♠ denied a balanced hand, 4-1-3-5 for instance.

The trend is to rebid 1NT here on all 11-13 balanced. The case for that is greater opposite an unpassed partner.
Probably doesn't matter much at IMPs. This seems to be a matchpoint issue.

[DavidKok]

I am not sure it is an issue at either form of scoring. Bidding 1♠ would win here, but bidding 1NT simplifies the auction opposite many other hands partner might have. It's just a balance of probability. You'll have some losses no matter which you choose, so pick the style that gains most on average (which I believe to be to bid NT with balanced hands at the earliest opportunity). At MPs the wins and losses are higher impact each, but I'm not sure the average flips sign.
As a minor aside, in the AEC-style Precision either the full relay 1♥ in the full system, or the simpler natural rebids in my custom version, have enough space to resolve the balanced hand and show the spades. My version is wasteful with bidding space, but there's actually room to play '1♠ = 11-13 balanced with 4 spades, 1NT = 11-13 balanced with at most 3 spades'. This is because e.g. 4=1=3=5 can rebid 2♣ to show '11-15 5(+)♣, 4♠, at most two hearts'.

[pilun]

Yes, one problem with rebidding 1♠ on a balanced hand is finding responder with a balanced 11-count.
Over a 1NT rebid, that 11-count has an easy pass.

A 1♠ rebid could be a shapely 14-15 count, in which case that responder will feel too good for 1NT and maybe try 2NT.
Either that, or opener will feel like raising responder's 1NT with a max.