Well there are NO guarantees either way you go unfortunately and that just sucks!
My story is the same as yours almost. I had tongue cancer a year ago and lost 25% of my tongue and had a left neck dissection. Lymph nodes were clear and I did not need a rebuild. But, I did have to have another surgery right after the first because one of the margins was found to have three 'stray' cancer cells. They took an additional 2 cm of tissue from that side and it was clear. They recommended radiation to me because of the 3 cancer cells that were outside of the tumor.
I went through with radiation. And as I sit here, I was diagnosed a few days ago with breast cancer. It's the same exact cancer my mom got when she was 50. Now, was there a correlation between the radiation and this cancer? Or would it of happened no matter what? No way to know for sure but I don't regret the radiation because in my heart I knew I was following sound medical advice and doing what I could to not have a recurrence.
Sorry, I'm sure this doesn't answer anything for you but I thought I'd share my story along with the rest.

Good luck in your decision!