The best implants for you are the ones that give you the "look" you wish for. Unless you gave me more information about yourself (that's what a consultation, personal examination, and complete surgical measurements are for!), a base diameter (did each of your doctors measure the same way I do?), present cup size ("not even an A cup . . . completely flat with some loose skin"), and stated goal ("a nice 'C' size") are all very helpful but incomplete. You have done a great job getting 2 opinions, and hopefully I can give you some more help without the direct examination an in-person consultation would offer.
First of all, your goals are the most important factor in helping you achieve your desired result. If a 4'10" tall 90 lb. woman with no breast tissue of her own wants to be "Dolly Parton" in size, anatomy simply won't allow that. This is pretty obvious to most patients. Similarly, a 6'1" 145lb woman who has breast-fed 4 children and had DDD-cup breasts when breast feeding, now has deflation and droop, and wants only to be a "natural-looking B-C cup" may need a full breast lift and only small, moderate-profile (widest implant base diameter) implants to achieve her goals--many of these patients want only breast implants to accomplish that, and don't want the scars or cost a lift mandates. Here again, anatomy simply won't allow that.
So, the degree of loose skin you have, as well as nipple position, may have an impact here. It sounds as if you don't require a lift, but a physical examination and measurements would confirm that in a moment. Let's proceed as if this (no lift needed) is the case.
Your final breast volume depends not only on the implants you and your surgeon choose, but also what tissue you "bring to the party." Based on your description, let's also assume you're coming to your own party pretty much without a contribution other than the skin brassiere. Perhaps you have had one or more children and may have breast-fed; this would be a very common situation with your described anatomy. In other words, you have very little of your own breast tissue, and your final size will depend mostly on the implant volume. I'm not sure what your mind's eye thinks a "nice 'C' size" is, but photographs from the internet or adult magazine can quickly show what you like, and what your surgeon's goal for your should be.
In over 2 decades of doing breast augmentation, I have learned that it takes about 250cc to equal one cup size in volume, so I would START looking at implants in the 450-550cc size range--perhaps a bit less if you have a little more breast volume than you give yourself credit for. Try on implants in a bra and stretchy top and see what you like. Then understand your surgeon should add 50-100cc to the size you choose to best match the look of the implants in a bra and top, since the under-the-muscle position takes away from the apparent final volume a bit (around 10% or more). Don't even look at the numbers, or, for that matter, the cup size label on your new bras--they will differ depending on manufacturer, and it's a LOOK you are going for, not a letter on a label (or cc's volume of implant)!
The most common reason for re-operation is request for larger implants, so you CAN avoid this mistake and get the look you want IF you do the following:
Bring one or more photos of desired breast size (unclothed, no bra, upright).
Try on implants in a bra and stretchy top and try not to look at the numbers, since your surgeon SHOULD make an adjustment in the operating room based on your tissues and anatomic findings at surgery.
See a couple more ABPS-certified plastic surgeons, particularly ones with years of breast augmentation experience. I suspect that both of the choices offered to you thus far are too small and will not meet your expectations. Of course this depends on numbers 1 and 2 above first, but based on what you have told us, I believe both may be off by at least half a cup size.
Read my article on the About tab on my Profile page on this site titled: "What is the Right Breast Implant Size for You?" or click on the link below for more information. Look at our photo gallery of other patients and see what implant sizes gave them the look you see. Our site features EXACT implant information, but some sites may offer what the patient or doctor "recalled" rather than actually reviewing each patient's chart as I did on our site.
It may be better to delay surgery and do things right the first time, rather than simply letting momentum push you forward. There is a reason you sought additional information here--listen to your gut or intuition! Best wishes!