How Are Under Muscle Saline Breast Implants Different from Silicone?

The muscle provides coverage of the breast implant superiorly and medially. Inferiorly and laterally the only coverage is the patients breast tissue and the skin. So yes over time you wil be able to tell the difference between the feel of saline and silicone breast implants. At the beginning you will likely not be able to tell the difference but once everything settles in you will be able to see there is a difference even if placed under the muscle.
Having said that, you are out of the age range for silicone implants unless you have a congenital deformity of your breasts. If not then at this point I would not worry about silicone as it is not an option for you anyways.
Good luck.

The silicone implants are unfortunately restricted until age 22. However, there are some differences between silicone and saline implants.
With saline implants you have more control over the volume you put into the implant. Saline implants have a range of volume and I think it is better to fill them to the maximal allowable volume. If there are differences in size between you breasts you can make up the difference with saline implants. With silicone implants, the volume cannot be changed.
Silicone implants also require a large incision whereas saline implants can be placed with a very small incision. Silicone implants do feel more natural and some surgeons feel like there is a low change of visible wrippling of the implant.
In your case, you have plenty of breast to work with which would minimize any difference betweent the feel of the implants. Also, putting the implants under the muscle would give you even more coverage, making saline a very good option for you.