If You Are Getting 350cc What is Different Between the C and D Cup if Volume is the Same?

In order to make an accurate size recommendation, I would need to assess your chest wall and breast mound measurements and characteristics.  Unfortunately, there is not a general rule of thumb or objective criteria to implant selection.
Your plastic surgeon will perform several measurements of your chest wall and breast anatomy and determine a range of implants that both fit your chest wall and reach your desired goals.
The next step is to try on this range of implants in the office with your doctor.   The key to this success is showing your surgeon the body proportion you desire with a bra sizer and allowing your surgeon to guide you to the right implant.   It will be much easier to communicate in implant cc's than cup size when determining the appropriate implant for you.
I wish you a safe recovery and fantastic result.
Dr. Gill

If you like how you look, don't worry about what size your bra is.  Bra sizing is very nebulous and inaccurate. Oprah has done shows about bra sizing and has shown that 50% of women in the US are wearing the wrong size bra.  Implants don't come in cup sizes.  Your bra strap size after augmentation is determined by the diameter of your chest wall directly under your breasts (this measurement won't change with breast augmentation).  This measurement plus 5 inches should be your bra strap size.  So if someone has a 30 inch chest-wall diameter, they would theoretically wear a 35 bra but no one makes odd-number bras.  So that person would likely wear a 34 bra sometimes and other times a 36 bra.

The cup size is determined by how much breast tissue you have plus the size of the implant.  The same letter cup size changes with a change in strap size.  For instance, a 34D cup will be similar in size to a 36C.  

Find a surgeon who has VECTRA 3D software which can help you visualize yourself with different size implants.