To me, a 3DS is definitely worth it for the first-party titles alone. I love mine. I've already sunk 200 hours into Pokemon X. And there are many more... Mario Kart 7, Animal Crossing: A New Leaf, Super Mario 3D Land, Bravely Default... So many good games. Also, Super Smash Bros will be coming out for it this year, which I am very excited for.
As for the model of 3DS, it really depends on your budget and what you need it to do. But if you do have the extra $30, I definitely advise you to get the XL model. It has better R/L buttons (the ones on the 3DS are exactly like the ones on the DSi, they are HORRIBLE.) It also has bigger screens, obviously. Another difference are the frontal SELECT/HOME/START buttons, which are now all individual buttons, rather than being compressed into one weird button. You might think portability is a problem, but it's really not too bad to just slip it into your front pocket. I wouldn't see myself running with the thing in my pocket, though. :)
If you want portability, buy a 3DS. If you want a cheaper device, just find a good used 3DS for about 80 bucks on Ebay or something. It's actually cheaper than buying a 2DS, and it still has all the features you want, like a folding screen.