Are subglandular or submuscular breast implants perkier? Which will "stay up" longer? I have a fear of sagging.

Thank you for the question.
There are pros and cons to the placement of breast implants in the “sub muscular” position versus the "sub glandular position”. I will try to outline some of the differences here; you may find the attached link helpful as well.
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.
Best wishes.

I prefer the subpectoral position (behind the muscle). There are several advantages to this. One, you will avoid the "ball in sock" appearance. Secondly, you will have more natural appearing cleavage. Thirdly, you will have a decreased risk of developing a capsular contracture. This is the development of too much scar tissue forming around the implant which may cause the implant to feel firm. Fourthly, you will have a better mammogram when you are older. If you are young with good breast tissue, I would not place your implant in the subglandular plane for those reasons. Hope this helps!