1. Ben senin için çalışıyorum. The exact translation for the first sentence is "I'm working for you." 2. Ben sana çalışıyorum. But the second one is more informal. You hear it especially on the mafia films. It has also contains the meaning like "belong to someone". By the way if you use the second sentence with "ona" but with meaning for like "lesson" or a "subject", it become also a common sentence, like "Ben matematiğe çalışıyorum." it is really similar to "I'm working math." just with the directional suffix. If you say, "Ben yemeye çalışıyorum." It means that "I'm trying to eat". I hope that it helps. İyi çalışmalar.

For me, the first one is more appropriate and sounds more natural in spoken language.

Ben senin için çalışıyorum. Daha uygun. Ben sana çalışıyorum da olur fakat daha kaba bir ifade.