This is easy to answer with yes/no, but hard to do so in any useful way. The truth of the matter is that it really depends on personal factors. Let me try to help you make that decision based on those factors instead.

I'll ask three questions. Pick from one of the answers below, and assign the respective value to each question:
Yes = 2.
Maybe / kind of / so-so = 1.
No = 0.

Questions:
Did you like the anime?
Are you interested in knowing more about the parts that got cut from the anime?
Are you okay with spending game money on VNs?

Now add the answers and evaluate the result:
5-6 -> definitely worth buying
4 -> worth buying if sufficiently cheap to your standards of cheap
3 -> hard to say, probably not worth it, but might be worth it if even cheaper than on the line above
2 -> only worth buying in a bundle with more interesting stuff
0-1 -> not worth buying, move along

As a matter of fact, the same scheme from above applies to Clannad (I don't have it on Steam, so I can't vote), by and large. I'd consider Clannad overall a little less worth buying than S;G for any given result, but that's definitely a point of contention; others would say the exact opposite.

I hope this helps! :D

It contained more content and was thus far more enjoyable for me. I watched the anime after playing the visual novel and the anime was personally too bland with too much content left out, honestly speaking.

The game is much-much better IMHO. I watched anime after playing, and while I've enjoyed spending more time with the characters it was a constant inside cry "how could you possible leave that very important part behind??". My biggest gripes with it, is that they cut out almost all the "sci" part of the "sci-fi", and mostly kept to the drama. I mean, I get it, they need to keep the story moving and the tension high, and all the relationship conundrums is better suited to keep people's attention than half an hour lection on time travel theories, black hole varieties or the way people's brain function. But I loved that in the game, how much they grounded fiction on real science, blending the line where sci turns to fi. All the theories, guesses and explanations they were coming up with, debates and discussions...

But even if you fine with more fast-forward storytelling and basic explanations, the human aspect is also greatly reduced. And if the main trio doesn't suffer from it too much, the rest of the cast comes out more flat and stereotyped than they deserve. You miss a lot of interactions, background and depth, not to mention their personal endings, which are emotional and diverse, and cover very different resolutions of the whole situation, both emotionally and scientifically.

Also I much prefer game artstyle.

Sorry for the long wall, and I don't want to be mean to the anime - I've enjoyed it. It's just that I love the game too much, it left too strong of impression on me. So now I can't shut up about it, and you provided a pretty good occasion for my affection-venting ^_^

so you couldnt enter?

able to pass now?

Rule has been changed

so if you enter and win, I've to request a reroll u guess.

Sex Drugs and rockn roll XP

Well... I'll give away some more games in the future. i guess you might like some of them as well ^^