As much as I love Brindleton Bay that comes with Cat and Dogs, and all of its build mode items, I'm going with Get Together also. Windenburg is a lovely location with a lot of options, and a lot of good stuff for interacting with other sims. Plus new things to do when going out and about. I've found that Get Together also actively does encourage you to have your sims go out on the town more than otherwise, which is good for players like me, because I tend to get so focused on the household goings on that I forget that there are so many places to go.
That said, if you do want the addition of cats and dogs to make your family play feel more realistic or complete, by all means! The town is beautiful, the build items and CAS items I think are absolutely fantastic. Cats and Dogs is worth it for that alone, to me at least. But if you want to focus on interactive things that don't revolve solely around pets, then go for Get Together.
I also absolutely LOVE City Living. More than I do Get Together or Cats and Dogs.