Why is your gut telling you to get a second opinion? Do you not trust the onc and the treatment plan, are you wanting to make sure all bases are covered or are you hoping for a miracle? Second opinions are important and I always think they are the best route even if you are happy with your current treatment (peace of mind if nothing else). But you must be prepared for the answer to be the same as the one you have now.
I only partly agree with the idea that your treatment center is the most important consideration. There are a lot of "for profit" centers that don't offer anything more in the end than a bigger name, better PR department and greater distance. Do your research on their results--both the results they tout and the results offered by outside reviews.