I prefer wheel controls over sliders and buttons. With a wheel control, you can have a greater range of adjustments rather than the small level of adjustment with a slider. With buttons, the volume could be too low or too high in between two adjacent levels.
In my opinion, I would prefer that Nintendo put louder speakers in their devices. For me, the 3DS XL speakers are ok, but if there is anything else making noise in the room, I can barely hear the game. The 3DS was barely loud enough to hear regardless of the situation, even in an inclosed room with no other noises. I never played games without headphones on.
I was happy with the volume of the DSi XL. I could hear all the games fine, even in a busy room, so I was disappointed when I got my 3DS XL.