The choice in practice
The Vanguard is the paddle people mean when they describe the Selkirk feel: a connected, quiet response that tells you exactly what you did to the ball. It is a control player's paddle and makes no apology for it.
The Luxx is faster and firmer. It sits closer to what the rest of the market moved toward, and it is the sensible pick if you have been eyeing a Six Zero or a CRBN but want Selkirk's build quality and forgiveness.
Who should not buy either
Both are premium paddles priced accordingly even secondhand, and neither solves a technique problem. If you are losing points because resets pop up or drives sail, a different Selkirk is not the fix and the money is better spent on court time.
If you want maximum power, neither is the paddle. The Luxx is the firmer of the two but Selkirk as a brand is built around control, and a player chasing pace will be happier on something built for it and disappointed by both of these.
And if you are buying your first real paddle, the SLK line exists for exactly that and costs a fraction of either. Buying a Vanguard to learn on is not a mistake, but it is not the upgrade people expect it to be until the rest of the game catches up.
Which is the better used buy
The Vanguard, for most people, and the reason is risk rather than performance. Its earlier runs are not thermoformed, which removes the one failure mode that makes secondhand buying a gamble. There is no perimeter to separate.
The Luxx needs the tap test, and Selkirk's high build quality is exactly why that matters: failures are less common than average, which is how an assumption of soundness lets an unsound one through. Check rather than trust the badge.
On both, edge guard scuffing is the most overweighted defect in the market. It should move the price a little and the grade barely at all.