My opinion..Cottage Living. It's at the top of my list if I were to rank the EP's in order of how much I like them. Meanwhile, Get Famous is literally at the very bottom.
For me, Cottage Living adds gameplay elements that I enjoy, which means I will make some use of it in practically every household that I play. Some of the biggest additions, like the ability to raise chickens, cows and Llamas are a huge favorite for me, but obviously not every sim I play will do that.
Changes to gardening, and the addition of oversized crops, however, will effect most of my households because I do gardening, to some degree or another, in most of them. Even if it's just to raise a few tomatoes on the apartment patio. And the addition of the Lot trait "Simple Living" which makes cooking require ingredients (as in you can't cook something without them) is something I WILL turn on in all households I play, because I personally thought the game should be that way by default. I want to require my sims to stock groceries.
The addition of Wild birds, rabbits and foxes are also cool, especially the birds and rabbits. It's just a cute addition to the game.
The new world is also incredible. Easily one of my favorite worlds in the game. The build/buy has some very charming things in it, and as "country boy" myself irl, those additions will get a lot of useage, because they are just the sort of things that appeal to me.
Get Famous, on the other hand, is all gameplay elements that I actually actively avoid unless I am playing a sim that I specifically want to make into a superstar. I have the entire fame system set to default to "off" for all new sims. I only toggle it on for the specific sim(s) that I want to make famous.
I also find the random walkby's of famous NPC sims to be an annoyance more than anything, because they cause all the other sims (including the ones you control) to just randomly drop whatever they are doing to run over and gawk. I use a mod to insure there's not a lot of these famous NPC sims in my games.
The world for Get Famous is also pretty dismal, with very few lots and nothing particularly interesting. Again...I pretty much only visit it if I'm playing a sim that is a superstar, and occasional to visit a club or restaurant there. And the build/buy for Get Famous is just not as appealling, to me, as most any other EP. Lots of Glitz and Glamm...just not my thing.
So...yah, I have an opinion on which is better. I could disable Get Famous and go a long time without even missing any of it. Cottage Living, on the other hand, I would miss immediately. Sorry for this post going so long...I guess I have strong feelings on the topic.