honestly, stats (combined with affection) have a huge influence on the power, hence another reason why having 40 6* girls is the endgame target (if you can do that) if not having at least all bloomable 5* girls... goes double for 2nd affection, and i got a lot of girls that don't have that... with ampy increases being small but adding up over time (cause you can give 3k HP, 1k atk, and 300 def to each girl and that times 40 is a lot)
but at the same time, equipment (especially the event ones) can give a nice boost to power when capped, and unique equipment just adds to this.
if you ask me, do either one first but eventually do both. But leveling girls seems to be easier compared to equipment cause equipment gets painfully slow when you don't have the bronze/silver/gold materials. if you know where to grind, then apmys and bond crystals for 2nd affection gifts gets trivial too.
but that's just my take on it, especially considering i'm still sitting with more than half of my best girls not fully leveled (and some can even bloom on top of this).
interestingly enough, you can grind materials and stuff for both knight leveling and their affections, but not equipment outside of the 1 time buy stuff costing well over a total of 100k medals at this point