MacBook Air's run hot when doing anything graphic intensive.. like 3D Games.. not sure how Sims stacks up, but I doubt it's as intensive as Eve Online, so likely fine.. And if more powerful 3D games are not on the agenda, then they are amazing.
I would strongly suggest the 13". I have the 13" base model from just about 2 years ago, with the RAM upgraded to 8GB.. never had an issue. I used it at collage, running multiple Virtual Machines, all day, without a problem. It's got an amazing battery life, and it light as a feather. The SD card slot on the side is a great place to slide in a card and leave it for extra storage.. Keep a few extras with it for even more storage, without the need for a full External HDD.
I saw and used a bunch of my classmates computer during the course, and end of the day, the Air was best bang for the buck. The price for the hardware isn't as overpriced as some would like to make you believe, and the fact it comes in such a light package makes up for anything else. It weights less than the battery from my PC Laptop.. lol
Thanks for the vauable information. So the 13" seems like the best choice to go if you say so Also I wasn't aware you can upgrade the ram in these machines due to how thin they are.