Moderate plus versus moderate plus extra?

Thank you for the question.  All of the FDA approved breast implant manufacturers provide good products.  The specific breast implant (Mentor's "extra") you are referring to as a higher "fill volume" of cohesive silicone gel within each implant.   For some patients, depending on how much soft tissue "coverage" they possess, this extra fill volume may help prevent problems with breast implant contour depressions and breast implant rippling/palpability…  For many patients, who do have a moderate amount of soft tissue breast coverage, the two breast implants you mentioned do not make a "big difference" in my opinion.

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.

Yes, MP Extra will give a little bit of projection for the same base width.  Cup sizes are not standard across bra makers, and implant sizing is NOT based on height, weight, BMI, or desired cup size.  This is something you will need to do IN PERSON with an experienced surgeon to determine what works well for you, and to get advice about if what you like is possible and what to expect with regards to results.  Be sure to discuss your goals and concerns with your board certified plastic surgeon and consider the pros and the cons. Be certain your surgeon is properly certified.  I've attached a link where you can start your search.  

Hello and thank you for your question.  The
best advice you can receive is from an in-person consultation.  The size, profile, and shape of the
implant is based on your desired breast size/shape, your chest wall
measurements, and soft tissue quality.  This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.   Make sure you specifically look
at before and after pictures of real patients who have had this surgery
performed by your surgeon and evaluate their results.   The most
important aspect is to find a surgeon you are comfortable with. I recommend
that you seek consultation with a qualified board-certified plastic surgeon who
can evaluate you in person.
Best wishes and good luck.
Richard G. Reish, M.D.  FACS
Harvard-trained plastic surgeon