Submuscular or subglandular if having a lift? My breasts are small and have 5cm droop after breastfeeding. I'm 40 years old.

I think submuscular is superior in almost all situations. Since you say you have thin skin upper chest then that makes submuscular placement even more important to have adequate tissue coverage over the implant. Discuss with your plastic surgeon. Good luck.

There are pros and cons to the placement of breast implants in the “sub muscular” position versus the "sub glandular position”. 
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) that can be seen with breast implants placed in the sub muscular position. 
Best wishes.

For the lift I would suggest a circumvertical approach. For implant placement, I prefer a true submuscular pocket, with the implant under both the pectoralis major and serratus anterior. I see many patients with subpectoral or dual plane who develop inferior lateral migration of the implant.
A subglandular pocket is less painful and looks good initially, but is much more likely to have capsular contracture, numbness to the nipple and inferior migration. It may present problems with mammograms. Since you say you have thin skin, you may be at risk of rippling of the implant in a subglandular position.