Ok, so, while these two seem completely different in terms of ideas, the fact is, they have some matching core aspects.
For starters, both are essentially 'slice of life'. That is to say that while there are some strange things that happen, not much of it is outside the realm of possibility. There's a bit of time travel stuff in Steins;Gate: Oukoubakko no Poriomania and there's essentially a second world in Clannad (which plays a role in Clannad: After Story, until which it can be used to see a metaphor for what the main character is feeling).
They also both have a kind of romance 'undertone' (in lack of a better word) to them, which both series leave till the end to really focus on for a while before ending it off.
They also have a bit of comedy in them but don't allow that to be the centre of the anime, that is to say, while there are some funny parts, if you're planning on watching either one for the humour alone, you will be disappointed.
One thing that I CANNOT stress enough here though, is that you should NOT watch Steins;Gate: Oukoubakko no Poriomania unless you've seen Steins;Gate, BECAUSE YOU WILL NOT UNDERSTAND WHAT IS GOING ON!