After pregnancy and lactation, many women, like yourself, are left with relatively floppy breast tissue and skin. In evaluating the breasts, I measure the breasts and notice whether the implants have "dislocated" off the rib cage. If the implants have dropped "down and out", often suture repair or capsulorrhaphy is required to "trap" the implants on the front of the chest wall. There are some other techniques, which are in development, which include the use of biologic material, which "stabilizes" the implants, much like a hammock.
After providing "book ends" for the implants, I prefer implants which are textured, so that there is less chance for the implant to drop again. Textured implants are less popular in the US, however are widely used, abroad. In making a tight pocket and using textured implants, the idea is to prevent recurrent sag. The mastopexy techniques have improved significantly but the unifying principle is to use an implant, whose diameter approximates your chest wall.
Finally, post-op management is crucial to avoid relapses from the suture repair. I generally request that women avoid chest exercises for 6-8 weeks and wear a snug exercise bra, 24/7 for the same period of time. We don't recommend massage or displacement exercises until 2 months. I hope that this will help your decision making process. Remember you will sacrifice some mobility for stability but the "girls" will stand high and tight. Good luck.