I would keep the original 3DS unless you can afford an XL. The only reason I have both sizes is because my Pikachu XL was a gift. Otherwise I would've been fine playing on the original.
If you want to waste your money be my guest lol At first i was thinking to trade my 3DS in for a Animal Crossing New Leaf and 3DS XL bundle but it wasn't worth it. It's too expensive.Just use your normal 3DS >.<
I traded in my purple 3ds towards the AC:NL 3DSXL bundle as well as a few unused games. Ever since I saw the bundle released in Japan, I've fawned over it, as soon as I heard it was coming the NA, I went and preordered it.
I did the same thing as you; bought a 3ds at launch and then hardly used it while waiting for new leaf.
That being said, I do not regret upgrading to an xl. The console was just everything I had expected from the original 3ds while being a whole lot easier to use for me. It's not a necessary upgrade, nor a cheap one, but I've found that I have gotten more out of my xl in the few months since I bought than I ever got out of my 3ds.
My advice is that if you see a really good deal, make the upgrade. Don't trade in your 3ds to gamestop, eb games or whatever though. They will just give you a shifty deal and make a fool out of you.
If the ONLY game you are going to play is new leaf, don't get the xl. Put the money towards something else instead.