Mentor Vs Gummy Bear Implants above the muscle to fix sagging in breast around 485-500cc?

 I would suggest that you be careful about the plan to "correct start sagging" utilizing breast implants.  Although it is true that there are cases where breast augmentation can do a very nice job in this regard, if breast lifting is truly necessary you will likely be disappointed with the results of breast augmentation surgery alone (regardless of location of breast implant placement or type of breast implant used).Also, make sure you carefully consider, the short and long-term pros/cons associated with breast augmentation done in the sub muscular versus sub glandular position. You will find a lot of information regarding this topic on this website.
I think it is in the best interests of most patients seeking breast augmentation surgery to have implants placed in the “dual plane” or sub muscular position.  This positioning allows for more complete coverage of the breast implants leading to generally more natural feel/look  of the implants in the long-term. This position will also decrease the potential for rippling and/or palpability  of the implants (which may increase with time, weight loss, and/or post-pregnancy changes). The submuscular positioning  also tends to interfere with mammography less so than breast implants in the sub glandular position. The incidence of breast implant encapsulation (capsular  contraction) is also decreased with implants placed in the sub muscular position. 
On the other hand,  sub glandular breast implant positioning does not have the potential downside of “animation deformity” ( movement/ distortion of the breast implants  seen with flexion of the  pectoralis major muscle)  they can be seen with breast implants placed in the sub muscular position. 
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. For example, 
 For example, I have found that the use of words such as “natural” or "D or DD cup” 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. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
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 breast augmentation surgery concerns) helps. Best wishes. 

Thanks for this interesting question.  Most US  based surgeons prefer #dual plane (partial submuscular/subglandular) position for improved upper pole soft tissue coverage, lower rate of capsular contracture and better mammographic screening.  Our South American (Brazilian) colleagues have demonstrated the subfascial plane (which is not the subglandular plane) but pair this with small implants, conforming to their patient's aesthetic ideals.  That being said, your objective is to "correct slight sagging" and the anatomical cohesive gel implants' lower pole arc provides the illusion of "lift" for individuals who have either pseudoptosis or lesser degrees of mammary ptosis (droop).  Obviously higher profile anatomical cohesive gels have more exaggerated lower pole arcs, which can be used to "redrape" the inferior pole skin laxity.  Unlike a formal surgical lift, these implants will not produce a round shape or allow significant superior positioning of the nipple-areolar complex.  The availability of the full range of different profiles, volumes confer Mentor a competitive advantage over Sientra, which has had to "ramp up" their production.  Do consult with PS who have extensive experience with the anatomical cohesive gel implants so you can assess which plane would be preferable .  The term "gummy bear" gels are attributed to Dr. Grant Stevens' experience with the Natrelle Style 410 implants, not Sientra.  Good luck

Hi AK
My bias is to put implants below the muscle because this position tends to minimize rippling, reduce capsular contracture, and minimize palpability of the implant.
I also strongly disagree with putting the implant above the muscle using the argument that somehow this gives more uplift to fix a sagging breast, perhaps even obviating the need for a breast lift with its attendant scars.
The reason the breast "sags" (I prefer the term ptotic, which essentially means the same thing, but without the negative connotations) is that there is loss of supporting structures, stretching of skin and supporting structures, and in general, loss of volume after child rearing.  
The Pectoralis Major Muscle tends to be very strong, even in women who are not athletic.  When the muscle is above the implant, it acts as a strong sling or (my metaphor here) an "internal bra" to keep the implant and breast up and supported.  Nothing will last forever, but I have seen many times over many years that implants/breasts hold up much longer under the muscle.
I have shied away from Sientra Implants, as I have for years been happy with Mentor and McGhan (currently favor the latter) implants.  Either Mentor or McGhan would serve you well, and it would be best to make that decision with an experienced board certified plastic surgeon.  Doing research is very important, but I believe that it is very important to come to consultation with an open mind, and work with your doctor to arrive at the best treatment for you.
All the best.

sek