I got bored with ANB really quickly, but I'm still liking SoS so far, though I just got it during the eshop sale a week or two ago. There are definite differences, and I like the mechanics of the game a lot more. Farming is simplified a lot imo, so it feels less time consuming (e.g., you get to water an entire plot vs. one crop at a time), and the speed of which you can push your cows is a bit faster too (that always annoyed me in ANB omg...SO slow). I like the characters better as well. It feels like they have more...color to them. The characters from ANB were a bit dull for me. There also don't seem to be renovation plans like there were in ANB, so I feel like the need to hoard items isn't as vital, but you still get opportunities to edit your town. The manner in which you ship your products/items is different as well.
Overall, there are differences so I think it's worth getting. But I mean, it's not necessary if you have ANB and are enjoying it. I only got SoS because I got sick of ANB.