3 stars may be a bit on the low side. I think Cartagra deserves 4 and KNS deserves 3 but 3.5 isn't really an option. Both are very entertaining and give you a lot of story/great art/terrific voice acting for what you're paying.

Cartagra is the story I like better. It ties up all the loose ends, although in my personal opinion, there were 2 twists too many. There's no running around like a blind squirrel looking for nuts. The story is more or less linear. Since I played this one after KNS, I thought it was pretty cool to see Reiji's pal Shugo in action.

KNS for all the praise it gets, is kind of confusing to me. It has a lot going on and you're really thrust into the role of a detective. That means meeting a lot of people and trying to remember a lot of facts. They're stored in a handy notebook you can access, but if you're a non-native speaker like me, you'll have a hard time keeping track of all the names that start with M. You'll also need a guide because, as in real life, you aren't always faced with obvious answer choices. In fact, it's not obvious at all where you're supposed to go and where you're supposed to be to receive critical information.

All in all, I don't think the murder mystery genre is too appealing to me. But if you're a fan of this type of storytelling, you'll probably love both games.

Cartagra:

Didnt like how much assistance Shugo required... it was really frustating to see him walk past clues that could have let him solve things almost independently.
H-scenes were longer than they needed to be, making them annoying some times.

Despite that and other things, the story and its lenght was good enough to say this is not the average VN. Recommended.

Kara no shoujo:

Most of the flaws in Cartagra were fixed here.
H-scenes last just enough for the scene to not become an awkward spam over the womans's AH AHH AHHH ..... oh and this protagonist seemed to be way better suited for the job.
Despite that, another equally or even more annoying flaw appeared:

TOO MUCH DEPENDANCE ON BLIND CHOICES!

Almost all the routes, even the true one, required blind choices in plenty of unpredictable moments. Escaping from bad end loops was very hard.
If you dont have enough free time, you will probably either drop the game for some time or use a guide.

Regarding the story alone, it was very good as well. Both games were quite long and interesting. Price tag is well justified.