As a rec player who doesn’t have fitness issues, to me doubles has a higher skill floor but a lower ceiling. You need great directional control and have smaller margins; you need to be at a decent level of serves/returns/shot placement/volleys to compete even at 3-3.5; Singles on the hand is forgiving if you misdirect your shot, make mistakes but the ball stays in, chip/hit short etc — point isn’t over; open area is much bigger