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.