"I see" is more common. When you say "So I see" you are telling someone two things: 1) You understand, and 2) you can see for yourself what you have been told. For example: You and a friend are visiting a very large park, and your friend says "This park is huge!" You can say "So I see!" because you can see it this for yourself. Another example: You and a friend are playing with a very friendly cat. Your friend says "This cat is very friendly!" You can say "So I see!" There are similar phrase to this, like "so I hear" or "so I have learned." You say these when someone tells you something that you already know or can sense for yourself. I hope that makes sense!