So going through your question,
Both the answers are correct. So, to clarify what I mean,
In your question you have mentioned that integral(1/x)=ln(x)+ C;
It is this C that plays a major role in both the answers.
If you take the answer that MATLAB has given you and take LCM(Least Common Multiple) of the term inside the log function, you get
By properties of logarithms, we have log(a/b)=log(a)-log(b);
So the answer can be written as
ans= (log(3x+1)-log 3)/3;
When you split the terms inside the parenthesis you get
ans= (log(3x+1)/3)-(log 3)/3;
log(3)/3 is a constant and shows in your answer as a part of C (The constant of Integration).
Hope this helps,