Sword of Spirit
Legend
Because it has a higher chance to land a crit that results in 2Y. This approach assumes you add sneak attack to the first hit if TWF. If you aren't doing this, then your overall damage output drops pretty severely. You have a new case where your damage for the round is just X and it's replacing X+Y.
The more d20's you roll, the more chances to crit you get. That's going to increase the sneak attack damage over the long run.
Yeah, what I'm not understanding (and I think I am missing something) is how making 2 rolls when only one of them can possibly hit is superior to making 2 rolls when both can hit. It's still 2d20. It's the general rule of Advantage that making two identical attacks is superior to making one attack with Advantage. In this case, they aren't identical, but it's 2d6+mod compared to 1d8+mod. So Sneak Attack chances should be unchanged.
The only thing I can think of that might be making this counter-intuitive thing that's being claimed might be something like:
Two Attacks: First attack hits, you add Sneak Attack. Second attack crits, you can't add crit Sneak Attack because you've already used it.
One Attack with Advantage: One chance to hit, one chance to crit, but Sneak Attack is always paired with crits.
Is that what's going on?