Definitely Ren for both "better for Kyoko" and "better at their career".
*NOTE: My comments may contain spoilers from the manga if you are only watching the anime*
Although they were both Kyoko's childhood friends, the two of them treated her very differently.
Shou has always used Kyoko to be successful, and I don't think he deserves her at all. Although he tries to be nice to her after the commercial where they work together, I don't think Kyoko will change her mind so easily towards him. For his career, those who seek for fame only will not succeed. You should seek for ways to improve yourself as well.
For Ren/Kuon, he has been Kyoko's support since she was small. Kyoko treasured the blue stone he gave her and also received a lot of advice from him before starting to get popular. For career, I think Ren is much more professional than Shou, at least he will not challenge someone like how Shou knocked over the garbage can.
Kyoko should go with Ren!! Let Shou get what he deserves for treating Kyoko badly

-Kira