Well.?

20mm is stiffer, and 5mm

Not sure about the new RS 15mm Maxle Lite but surprisingly Fox’s QR15 is heavier than Rockshox 20mm maxle lite despite being an XC standard (*)

15mm should allow for either smaller lighter hubs, or bigger longer lasting bearings in the same size shell. In practice it tends to mean the same shells and bearings with a big adaptor which ends up being less good, but not always.

Oh and yes 15mm is less stiff though both are effective… But there’s a reason Fox use 20mm for 36s.

(* despite all the toss MBR keep spouting… They repeated the propaganda in an article that actually included the correct weights, that was good)

20mm is a perfectly good standard, that’s been around since the year oat-cake.

15mm is in all practical aspects exactly the same as 20mm, but of course incompatible.

and it cost $XXX,XXX to design, establish and advertise, and we’re paying for it.

clearly a sign that bike design has peaked.