I have both.
Not a similar sounding profile. Not even in the same ballpark in regard to technicalities or overall tuning. Truly apples and oranges.
The 5909 is Harman tuned meaning the lower and mid bass is substantially elevated and treble slightly "hotter" in the upper FR. Not neutral or "reference" tuned.
The Stellia is neutral tuning with a bit of a bass bump (not as much as Harman tuning). It's a much more resolving 'phone with better depth and width of soundstage.
The 5909 at it's core is a NC/BT headphone. It isn't meant to compete with the likes of the Stellia, DC Stealth, or Meze Liric. The fact they perform so well wired is a bonus, not a core feature. The Beryllium driver does sound much better than all the current ANC headphone drivers from the competition (I've owned several, but
@angelom observations cover many more). The Beryllium driver is faster with transients and overall more resolving.
If you like the Focal house sound and have the cash, Stellia all the way. If you want a pair of ANC headphones that give you the Beryllium driver goodness, the 5909 is great. Like I said in a previous post, with the 5909, the delta between using the Stellia is much smaller than with the other ANC headphones I owned.