Da Capo : Basically a highschool romance story about one boy and the women around him who all have something going for him. One day, Junichi's cousin and childhood friend, Sakura Yoshino came back from America all of a sudden. To Junichi's surprise, she looks exactly the same as the girl that moved away six years ago, and hasn't aged one bit. And she came back to remind Junichi of their childhood promise... She's still in love with him, but what about Junichi's sister, she seems to have feelings towards him as well. It is a bittersweet tale of magic, love, hidden desires, and unattainable dreams.
First half is rather light, most characters get introduced here and not too many heart breaking episodes. The second part however is very tragic and dramatic and something you can't stop watching. Also every episode has 15 minutes of actual story and 5-7 minutes of a filler episode/music video. just SKIP these completely and come back to them later, Absolutely nothing happens that is in any way related to the story.
Kanon : Boy returns to live with his aunt after being away for 7 years. He meets several girls he knew before but he doesn't remember any of them, the story revolves about current and previous things that happened between him and them. Heartwarming, at times depressing , a touch of magic but very beautifull.
Again in the first episodes we get introduced to all different characters but you have no idea what their backgrounds are or their connection to the main character. This seirs is very serious from the first episode. Story really gets going in ep 3 or so, the two ep before are introduction. Make sure to watch the first 5 minutes of episode one directly after finishing episode 13 !
To compare : Kanon starts seriously straight away, and is a bit confusing at first. Da Capo starts out lightly as a regular high school romance/comedy anime. The really dramatic KGNE parts only start between ep 12-15 but from that moment on it's beautiful. Also Da Capo is a triangle relationship just as KGNE was while Kanon never feels that way (this is due to the nature of the girls introduced but that is part of the series ).
I recommend .... both
but if you want pure drama take kanon first, if you don't mind a lighter start after which the serious part begins then pick Da Capo. If you really wanna have another triangle relationship like KGNE Da Capo seems the better choice.
PS While both are very good romance/drama I think the Kanon ending was better (but this is only my opinion)
PēS Keep in mind that contrary to very realistic KGNE these two anime have a (slight) touch of magic in it which is essential to the story.