Hi Federico!
When my 24" iMac croaked, I went with the 27" because I couldn't imagine going smaller. I like it for all the windows I can have open. Mine is from early 2012, so It came with 4 GB, and I updated it to 16 GB. I wondered whether the new style was user upgradable for RAM. Apparently not. Too bad, as my bump from 8 GB to 16 GB was about $100 USD.
I don't use any CPU intensive applications, but I do have a lot of applications open and running most of the time. Mail, Safari, iTunes, WeatherCat, Tweetbot, EvoCam, Growl, Dropbox, Meteorologist, and more background stuff are running all the time. With all of that, I seem to have 8-10 GB of RAM free most of the time. In my case, having 16 GB keeps me from having to open/close apps.
We just upgraded my wife's 2009 17" MacBook Pro from 4 GB to 8 GB, and it seems like a whole new machine. All she uses is Mail, Firefox, and Tweetbot, and occasionally Word and it was so sluggish you could barely change between apps. Firefox is a bigger RAM hog that Safari. No problems after the upgrade.
So I'm of the opinion that over the life of a machine, you're probably going to need more RAM than anything else. You can always get more disk space with an external, but if the RAM is not upgradable, get what you think you'll need 3-5 years from now.
I hope that helps,
Steve
Activity Monitor