I'm considering largest size silicone implant! Sientra 695cc vs. Mentor 800cc?

What is the biggest implant I can use for breast augmentation? That answer is simple: the largest implant they make.A much more important question that you should be asking instead is, "What is the biggest implant I SHOULD use for breast augmentation". This question can't be answered so easily and you should be very well informed by your plastic surgeon about the consequences of your choice.A few things you need to know and have hopefully already been informed by your plastic surgeon:The Mentor 800 cc implant is 15.5 cm wide. I have only rarely measured a patient in my practice that I would allow to consider this size implant. Based on your height/weight and description in my opinion this implant is far too wide for your chest wall and will significantly increase the chance of either symmastia (because your surgeon is trying desperately to force this wide implant to fit on your frame and overdissects the medial pocket) or the implant falling out to the side because the width of the implant forces its way through the weaker lateral tissue of your chest.Any implant of that size significantly increases the risks of the surgery; Increased chance of altered sensation, tissue thinning, bottoming out of the implant, rib deformation, and need for reoperation.As long as you are aware of the consequences of your choice then you can make an informed decision. Unfortunately sometimes plastic surgeons need to tell patients information that they don't want to hear but need to hear anyway. Yes, it is elective surgery and ultimately you can "elect" anything that you want, but plastic surgeons are doctors first and foremost and should supply you with the information regarding the risks.

It is very difficult to determine the exact size and shape implant you will require to best match your ideal breast image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your breast will determine, in many cases, the maximal volume per implant profile that you can accommodate. To illustrate; a 100 cc difference may make a significant difference with a narrow base width breast, but much less of a difference if you have a wide chest wall and wide breast “foot print”. Therefore, just because your friend may have a great result with let’s say a 350 cc implant to make her go from a “A” cup to a “C” cup size does not mean that you will have the same result with the same size implant. . Further simply placing implants in a bra to determine the size best for you is not always accurate as the bra often distorts the size, is dependent on the pressure the bra places plus the implant is outside your breast and not under it among other variables. Computer software morphing programs that automatically determine the best implant size can be helpful in some but not all cases (e.g. doesn’t work well in my experience with existing implants, sagging or asymmetric breasts). Using “want to be” photos however are useful if simply provided to the surgeon as I will further explain in the link below.

Thank you for the question.  
Sorry, but neither implant is "better"; one may be better when it comes to achieving your goals however. Unfortunately, online consultants will not be able to provide you with specific enough advice to be truly helpful.
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.
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 "D cup" or "fake looking” etc means 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.
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 ( after the use of temporary intraoperative sizers).
I hope this, and the attached link (dedicated to larger breast augmentation surgery concerns), helps.