As others have stated, silicone will feel softer, more natural, and more homogeneous with your own breast tissue. Saline implants will have sufficient tissue coverage in your anatomy for a good result as well, but you may have rippling, a "water-balloon" feel, and potential for eventual deflation requiring re-operation. (All to save a few hundred bucks?) 98% of my patients choose the newest cohesive silicone gel implants for a reason (and NO, you do not have to get an MRI every two years to check for rupture--these implants are cohesive and cannot leak).
As to size, this (and the saline vs. silicone issue) is discussed in detail on my website (link below) and in my article "What is the Right Breast Implant Size for You?" found on my "About" page. Realize that a 50 cc difference is 3 tablespoons and 1 teaspoon of volume, which you cannot even detect. Discuss this with your surgeon, try on implants in a bra (and then allow your surgeon to use 50-100cc more than what you choose), and show your surgeon photos of models or patients (unclothed) with what you consider your own goal--that way your surgeon can use your own anatomy and measurements to select the best size and implant profile to give you as close to ideal results as possible!