Which is better for breast reconstruction fat grafting or alloderm?

Neither: Many plastic surgeons, myself included, use acellular dermal matrix grafts (generic phrase for alloderm) and fat grafting. They are usually used at different stages of breast reconstruction though circumstances may dictate using both. Neither one is "better" as they often serve different purposes.

Many options: There are many approaches to breast reconstruction. Generally speaking the timing can be either immediate or delayed and the breasts can be "built" using either implants or your own body tissue. Often combinations of approaches are used. Alloderm (an "adm") is commonly used with implants. Fat grafting is a newer technique. Both have pros/cons. Consult with a plastic surgeon to discuss options.

Depends on procedure: Fat grafting and alloderm are two different things. Alloderm belongs to a group of products called Acellular Dermal Matrices. In breast reconstruction they are used to create a sling under the muscle to better position and support the implant. Fat grafting uses your own fat to either increase the volume of the reconstructed breast, or to smooth out the contours and edges of the implant below.