Should I get 300cc or 350cc Mentor rounded, smooth, behind the muscle implants with my lift? (photos)

What supports your breast is your own tissue which is stretched and saggy. Going smaller will be less weight and may hold up better. You could go with the size in between, the 325 cc implants. An internal bra might be helpful for you as well.

Thank you for the question.   
It is definitely worth spending more time (prior to the date of surgery) with your plastic surgeon to discuss further.  Show your plastic surgeon what you wish to achieve using as many visual aids as possible.   This additional communication will go a  long way when it comes to helping alleviate your anxiety prior to proceeding.
 Some general thoughts regarding this combination procedure may be helpful to you:
 Generally speaking, patients undergoing the breast augmentation/lifting combination operation should understand that it is significantly more complex than breast augmentation surgery only and that it is associated with a significantly higher risk of complications and need for additional surgery. 
Also, patients undergoing the combination breast augmentation/lifting surgery should understand that there are limits as to how large of an outcome can be achieved, given that breast lifting surgery does involve breast skin removal. On the one hand, breast augmentation surgery is expanding the breast skin “envelope” while breast lifting is (by definition) tightening up the breast skin envelope. These 2 forces are counteracting each other. 
Therefore, it becomes important to remove the appropriate amount of breast skin and to use the appropriate size/profile of breast implants to balance these 2 forces appropriately and to allow for achievement of the patient's goals while minimizing risks of complications. 
In my practice, "tailor tacking" is an important part of the mastopexy and mastopexy/augmentation procedure. It involves TEMPORARY  closure of the skin (for example with temporary skin staples)  to double check the amount of skin that is to be excised during the procedure. This maneuver is done to ensure that enough skin, but not too much, is removed. On the one hand, it is important that enough skin be removed to achieve the aesthetic breast lifting results the patient is looking for; on the other hand, is important not to move too much skin creating tension upon closure and potential wide scars.
 
Also, generally speaking, the best online advice I can give to ladies who are considering breast augmentation/lifting surgery (regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work. 
2. Again, have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining which operation and/or breast implant size/type/profile will most likely help achieve your goals.
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural breasts” or "C or D cup" etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup size may also be inaccurate. The use of computer imaging may be very helpful during this communication phase.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery. The use of temporary intraoperative sizers with the patient in the upright position makes selection of the best breast implant size/profile relatively easy.
Best wishes.

Thanks for your question and the photos. It appears that you are a good candidate for a lift. The benefit of an implant will be more fullness at the top of the breast and of course larger size. If you are looking at the same style of implant then a 350 would be wider than a 300.  I would recommend that you try on implant sizers with your PS. I focus on diameter of the implant so that the outer curve of  the breast balances with the curve of the hip. Too wide can look top heavy, too narrow can look torpedo like.Once we hone in on the width, the patients can choose how much projection they want, by choosing a low, moderate or high profile implant in that width.I have included a link below that I hope will help you with sizing as well.Best wishes.