Everybody has different tastes though and the 530 might be perfect for your needs. The 531 looks pretty sweet ain't gonna lie.
Yeah, so I didn't specify up front but this 530 vs 531 question is intentionally specific. I'm looking to fill a very specific niche and the 53x series is perfect. I'm supplementing my existing library. My go-to these days is still my 720S but this 530 is really best described as "my Benchmade Axis locking ultra thin replacement for an Kershaw Leek".
What do I like about the Leek? Metal handle. Ultra smooth and rounded edges (nothing to catch on clothes). I was able to get a $10 deep carry clip for it, too (although the clip isn't perfect).
What don't I like about the Leek? I hate the blade shape - super impractical for my usage (I want a heel, I want combo edge). The build quality is nowhere near a BM. I also really want Axis lock, which is really just because I love it not because it really matters in a thin knife like this (let's be honest, I'm never really going to crank on something this thin to the point where a solid liner-lock would release, but hey, I love my Axis!), and finally I really don't want spring assist.
Best case I'd fine a production 530 type ultra-thin knife with a drop point or bowie style blade (e.g. 531), an aluminum handle, an Axis (or similar), and a price tag <$100 (at least under $200 - I know I can get a collector edition 531 w/titanium but I'm not spending $500 on an EDC).
My original question still stands: how does the 531 handle's smoothness stack up next to a 530?
*Edit: I forgot to mention: overall thickness of the 530 is *perfect* for my current search. I'm fully understanding of the "thickness vs rigidity" tradeoff I'm making. While I'm at it the closed and open sizes for the 530 are also exactly where I want them to be.