V-Sync on or Fast with Sims 3?

That's interesting. i don't have that option. I us adaptive. You can try fast, adaptive, or vsync on. See what works best for the game and your card. You need to make sure your monitor refresh rate matches to your fps. Example If your monitor is 60 hz, your fps should lock at 60 in game.

You can use in game cheat...testingcheatsenabled true and then fps on. The game fps will show in game in the upper part of your screen . If the fps does not lock , there are other methods you can use. which @igazor or @SimplyJen can help you with

Fast Sync is kind of new for the more powerful cards. From what I understand, it's sort of a "lite" version of fps capping without forcing the game and the monitor to actually stay in sync at all times. I wouldn't recommend it for TS3 unless you are getting consistent and wild frame rates in the high hundreds or thousands and your system seems to be struggling too much to maintain vert sync when set to On or Adaptive. I suspect it's really more for first person shooter types of games where the fps rates can get more carried away during routine play than randomly fluctuating all over the place like TS3 can tend to do.

Fast Sync started with Pascal cards. (architecture developed by Nvidia) The 10 series cards have this option. It's for people looking to play with x2 their monitors refresh rate for high FPS games without the risk of screen tearing... The Sims is not a high FPS game. It's recommended to play with vsync On for this game locking it to your screens refresh rate.

testingcheatsenabled shouldn't be required to use the FPS on cheat.