I had one of my tension spring snap and had to replace it in a hurry. I thought to replace both garage door tension springs at the same time but the guys recommended that I didn't replace the other one as it seemed to be in good shape (despite some rust and neglect). It looked like it was replaced more recently (by a previous homeowner).
Save me some coin - they lubed and tighten the garage motor chain for me me as well. Fast and efficient same day service.