Low pair or 4 to an outside straight?
Keep the low pair. Exact EVs on 9/6 Jacks or Better: the pair is worth 0.8237 per coin vs 0.6809 for the open-ended straight draw — the opposite of the flush case, and the most commonly reversed pair of rules.
The exact numbers — 5♣ 5♦ 6♥ 7♠ 8♣
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
A straight only pays 4 (vs 6 for a flush) and eight cards complete it (vs nine). That smaller, cheaper draw drops below the pair's steady two-pair/trips/quads equity. People who learn 'draw beats pair' from the flush case leak EV here for years.
Chart line #9 of 18 (low pair) sits above line #10 (4 to an outside straight). See the full solver-optimized strategy chart, or drill hands with exact-EV feedback until plays like this are automatic.