What grade of ptosis do I have? Will dual plane implants suffice? (Photo)

Great question!  After looking at your photos, your nipple / areolar position is on the low side, one of them is folding over the bottom part of the breast.  I would recommend the breast augmentation along with a breast lift - you maybe able to get away with a donut lift which creates the scars only around the border of the areola and heals in very nicely.  However only with an in person examination can this be determined.Best of luck and hope this information helps!
- Dr. Roger Tsai  @DrRogerTsai
Board Certified Plastic Surgeon

I can certainly understand your hesitation with the scars that are associated with a breast lift, but given your nipple position in the photos as well as your asymmetry between the two breasts, I think you would have a much better result if you were to proceed with a lift along with the implants.  Remember that you are doing the operation to achieve breasts that are most aesthetically pleasing and while scars are always best avoided, sometimes they are necessary to achieve the results you want.  While you would be able to do an augmentation alone and return for a lift later if you are not happy, based on your pics I think a lift with the augmentation as a single stage operation would be much more likely to give you the results you seek.  Remember you can always get more opinions with an exam in person which can help you make your decision.  Best of luck!

Patient concerns regarding the appearance of their breasts can typically be divided in two pools: volume, ptosis (droop). The procedures to address these issues are as different as the issues themselves. However, these concerns are not mutually exclusive (some want smaller less droopy breasts and others want larger less droopy breasts).
There are two factors which can help to answer the question of which surgery will deliver the most satisfying result: mastopexy, augmentation or both. Patients must first consider the position of the NAC (nipple areolar complex i.e. the pigmented portion of the breast) as it relates to the inframammary crease. If the nipples are at or below the level of the crease the patient may benefit from a lift or mastopexy. The breast lift is designed to elevate sagging breasts to a more youthful position. It also helps to reshape and tighten the skin of the breasts. This is typically accomplished via a periareolar (around the nipple) incision with or without a vertical incision.
Mastopexy is an effective way to address sagging breast, however, the changes which accompany weight loss or pregnancy can also decrease volume of the breasts. When a woman wants the volume restored, or increased, a breast augmentation will also be needed in conjunction with mastopexy. Augmentation can help restore the volume loss which is well known to women following children. There are a number of options available to women considering augmentation ranging from size and implant type to access incision.   
A breast augmentation alone may be appropriate if ptosis is mild. If the patient is comfortable with the natural appearance of the breasts but desires them to be larger, a breast augmentation alone is the right procedure.  Augmenting significantly ptotic or sagging breasts tends to magnify the problem. In these cases, a mastopexy performed in conjunction with an augmentation is preferred.
With regards to your specific question, subglandular implants ("overs"), in my practice are reserved for those with significant breast tissue to conceal/camouflage an underlying implant or those in need of a lift but are unwilling to consider it (as this provides for a better ptosis match).
Based upon your photo alone, it is difficult to say for certain the grade of ptosis (this is best assessed via in-person exam). If your surgeon believes you are borderline, you may take a staged approach, beginning with an augmentation. The lift can be decided on at a later date if you are dissatisfied. 
There is a procedure for every problem. To determine which technique is right for you, consult with a board certified plastic surgeon (ABPS). Patients should discuss their concerns with their physicians in order to make an educated decision.