Which implant weighs more; Silicone or Saline?

Thank you for your question.  Comparing the same size (cc's) implants, the silicone ones will be lighter than the saline ones.  Because of your back problems, staying conservative with a smaller silicone implant will be the best option.  

With respect to your back problems, the size of the implant chosen will have more impact than whether you choose saline or silicone. Another factor is how firm your breast tissue is and whether you have any sagging. Breast that are large, but firm feel lighter than heavy breast that sag.The difference in weight between saline and silicone implants of the same volume will not have much impact on your back pain.