Number one isn't used often, however if it's used by someone it could be because the speaker is emphasising on the fact that whenever he goes to the other guy's home on the weekends, he finds him WATCHING tv at that moment, so basically emphasising on the fact that he sees him doing the action every weekend at the time he arrives there. The other sentence is the safer option for the student as it's definitely correct and widely used, but it just refers to someone's habits in general.

Number one sounds a bit odd and may make sense in some types of English but would not be used a great deal in the UK, it sounds a little archaic.

Per what Shiv says, second sentence is talking about something he always does whereas the first is more of something he's currently doing, even if he's doing it all the time.

The second sentence tells about habit. He watches TV. First sentence is telling about present time... Like these days he's watching TV. He watches daily or not, it's not sure. But the second sentence is telling about habit.