Is it worth upgrading to SSD or just to buy new?
I have a Late 2011 MacBook Pro too. After upgrading to Yosemite last year, I was thinking on buying a new Mac, since performance was very bad. 
Every kind of application was incredibly slow to open, even working with more than one app at time was impossible. Also just opening a simple PDF took a lot of time.

At the end, I decided to install a SSD on my Mac. A new world. Now everything works properly and my laptop seems to be reborn. I also updated to El Capitan and performance is even better.

So I advise you to buy a SSD, before giving up with your Mac. I have a Samsung SSD SATA 250 GB and I am very satisfied. You can spend less that 120-130 $ and you will have a completely new experience.