Body
Pro: The Avatar body in SL is more 3D and very High Definition (HD). This makes it excellent for realism in fashion photography and edits. The Avatar is animated to move naturally, and each body part can be isolated and adjusted with free system scalars to achieve your unique body and head shape. Mesh bodies, body shapes, Bento animations, Mesh heads and other mesh layers can be brought to enhance the basic system avatar. Bake on Mesh (BOM) is available to allow for easier layering. The Avatar body can be nude when clothes are removed. SL's age restrictions are 18 and up. Feet can be adjusted from pointed toe to flat on some Mesh Bodies.
Con: Clothing, shapes, heads and accessories are layered. Without (BOM) when adding new items or changing existing items you will need to detach worn items. As a "Noob" if not careful, it's very easy to remove wanted layers. Your head, body shape, and other layers can be deleted easily when dressing causing you to,at times, to start over with dressing and possibly rescaling. It can be a pain to find missing items in your inventory with no visual cues. Even skilled users agree, the SL inventory can be a challenge to navigate. Layers have to be specific to the Mesh Designer base. Make up and accessories are not always universal and building a "play ready" Avatar can be expensive. The difficulty is high/ advanced.
Mesh Heads:
Pro: Skins and makeup can be added. Skins for head and body can be mixed and matched. BOM is available with appliers. Mesh eyes and facial animations are available. Hair can come in two forms Flexi & Mesh. Mesh hair styles offers more hair colors per purchase. Flexi hair offers limited colors but realistic movement.
Con: Neck and body can be brought separate so necks seams show discoloration and at times when brought separate, the head skin doesn't match the body skin tone. Hair base needed. BOM merges layers, but can be challenging for newbies.
Second Life Avatars serve body ody ody better than IMVU!