I play in Fullscreen because it's the only option that let's me edge scroll. I can't imagine trying to play without the ability to do that. I've always played that way. It's interesting that y'all say Fullscreen lags your computer. When I was playing on my crappy laptop, both Windowed modes made my computer horribly lag, but Fullscreen didn't. I don't know if Windowed mode would lag the gaming desktop I have now (probably not, this thing is fantastic!), but I really don't have a need to try it and see.
When I'm playing my Sims, I play offline and immerse myself in the game. I get so into it, sometimes I lose track of time and before I know it, hours & hours have gone by and I've forgotten to eat. I've had to force myself to stop and get some sleep. Ah well, I only play when Hubby's sleeping, so it's all good. I can't play when he's awake. Besides the fact I want to spend time with him and get other stuff done, I can't stand someone looking over my shoulder when I play. I have no idea why. *shrugs*
Happy Simming! <3