What implants should I get? Polytech vs. Mentor Round vs. Mentor Xtra vs. Motiva Ergonomix? (photos)

Thank you for the question and pictures.  Based on your photographs, I think that you are starting at a good place and should have a very nice outcome with breast augmentation surgery.  There seems to be is a moderate amount of breast asymmetry and the use of different sizes of breast implants may help to improve overall breast symmetry.  All of the FDA approved breast implant manufacturers provide good products. Each of the manufacturers will use different terminology when it comes to describing their breast implants (and each will claim that they have the "newest" and "best" breast implants). 
 
Personally, I think that careful selection of plastic surgeon and careful communication goals are  the two most important "factors" when it comes to achieving the desired outcome with breast augmentation surgery.  The specific breast implant manufacturer and even type of breast implant are far less important factors, in my opinion.
 
  Generally speaking, the best online advice I can give to ladies who are considering breast augmentation 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. Sometimes, it is necessary to seek several consultations before you feel comfortable about your choice.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining 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” or "C or D cup” etc may mean different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, best not to discuss your goals and/or judge the outcome of the procedure performed based on achieving a specific cup size. The use of computer imaging may be very helpful during the communication process. The use of in bra sizers may also be helpful. In other words, use as many “visual aids” as possible during the communication process. I encourage patients to meet with me as my times as necessary, to feel comfortable that we are both on the “same page”.
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. I generally select appropriate breast implant size/profile after the use of temporary intraoperative sizers and viewing the patient's chest in the upright and supine positions.
 I hope this helps. Best wishes for an outcome that you will be very pleased with.

Your surgeon who has examined and measured you will be the best one to guide you to the range of sizes that best fits your frame and tissue. There you can try on a variety of sizes to see what you prefer. Mentor is a great company that has stood the test of time, stands by their products, develops safe and FDA approved implants, and gives a great warranty.

I only have experience with Mentor and Allergan implants. Both manufactures offer a number of different styles, sizes and profiles. Measurements taken by your surgeon will help him/her determine the best size for you. The most commonly placed implants are round smooth saline or silicone implants.  Best of Luck  Dr Harrell