My first 3ds was a 2ds (lols) but after playing on the 3dsXL I could never go back to the tiny screens on the 2ds or 3ds (they have the same screen size right?). I feel the controls are slightly more conveniently placed on the 2ds because you hold it a bit more like a normal controller, but it's still really cramped and not much better than holding the other systems. I know there's acceseorries to add grips to the 3ds to make it feel like a controller but I haven't seen one for the 3dsXL yet, but when I do all my woes will be gone.
Plus, the C-stick comes in handy for monster hunter when you want to move the camera around while running, frees up space on the touch screen for other functions. Z buttons can be used to access chat quickly or to quickly toggle which monster you have locked (though I do with it had a different function). If you use the 3d at all then the face tracking 3d is much more pleasant to use than what's on the 2ds.
2ds is cheaper but can't fold in so it's actually harder to fit in your pocket. Pros and cons.
I just double checked your poost and saw you're getting the old 3dsXL and not the new one. I played over 200 hours of MH3U on the 3dsXL so 120 dollars for the system is a pretty good deal. You just won't have quite as many neat features but it's still entirely playable.
EDIT: As for the new monster hunter itself it probably has the best singleplayer experience out of all the monster hunters I've seen (I actually got into monster hunter because of some Let's Plays on the LP Archive so I saw the singleplayer for some of them). Plus, you can play online with people which is pretty fun even with PUGs (I haven't encountered any trolls, myself). The addition of verticality in the levels is akin to the addition of underwater segments from 3u except completely awesome and not shitty. The game looks just as beautiful as any of the other MH games which is impressive considering it's on a handheld. Even so I would have loved to have played a game like this on a PS4 or something.