What Should I Replace my Saline Implants with to Reduce Sag Factor?

There is nothing with implants that will solve the sagging factor. Silicone gel implants will feel more natural in sagging breasts that have generally thin coverage over the implant but they won't look better or solve the sag issue. 

In the face, sagging tissues can look better by gaining weight but this is neither reasonable or desirable for breasts. A lift procedure is the only way to reposition the breast and the implant behind it to a higher level. The type of implant only changes the feel. 

Breast implants, no matter what type they are, can correct only a small amount of sag (what we refer to as pseudoptosis, i.e. the areola is slightly below the inframammary crease but the nipple is entirely above it before an initial augmentation). Any other sagging requires a lift of some kind to correct the sag. If you had the sagging before the implants, and I could not tell if that was so or not from your question, then what you need is a lift.

If you have sagged over the implant, you also need a lift. If what you have is stretch of the inferior skin so that the distance from the areolae to the chest wall has increased, this is a different, more complex problem. Again, the implant will make no difference. The only way to fix this is to revise and reinforce the internal scar around the implant, and this does not always work. You need to see a certified Plastic Surgeon who has significant experience with breast revision surgery.