New consoles have to be installed onto the console hard drive, regardless if you have the game disc (having the disc simply helps load the games faster). New games take up 40-50 GB each. Quite often, but not always 500GB systems come with a game while the 1TB doesn't (not always but sometimes).
If you're only going to be playing 3 games then a 500GB system which might provide a free game would be the way to go. If you end up buying more games and need additinal space you can always buy an external 2tb drive for 50/60$ (when on sale). Having said that, if $$ is not an issue get the 1TB system from the go (it's convenient).