First of all, how long ago did you take the old MCAT? And are you looking at MD or DO schools?
In reality, the advice for the classic "should I retake" question is always the same -
only retake if you think you can do something differently (from what you did on either of your previous attempts). Too many students try to retake and basically prep in the same way that they used to, or prep less because they've reviewed the material in the past.
In your particular situation, another question to ask is "why did you decide to take the new exam?" Did you worry that 30 wasn't high enough for your goal schools, or was there a reason why you thought the 2015 test would benefit you (for example, if you were really good at psych or biochem)? The next question is
why it didn't work out as well as you hoped. This is essential for deciding whether to retake - if you were sick or just had a major life event happen that messed up your score, retaking is more advisable; if conditions were great and it still didn't go as well, retaking might be riskier. Most retakers score the same as their previous score (and some score lower, of course), so it's important to be careful.
Assuming that your 30 on the old test was recent, I'd still lean toward retaking, but make sure you have enough time to prep thoroughly! And good luck
