Body
Pro: Mesh body's is 2D and comes in set proportions. When posed and in some clothing it can have a more 3D appearance. Scalars are available for changing Avatar shapes. Scalars range from thin to XXL. Dances, poses and animations can be added to create simple movement. Clothing is brought according to the mesh body type. Feet and ears can be adjusted with add-ons. Avatar can have Mesh or non mesh heads and bodies. Make up and accessories can be added as a layer. A "play ready" Avatar can be created for less money.
Cons: 2D body looks flat and can have a cartoon quality. HD skins are available but many skins look pixelized. Scalars and animations have to be purchased to change the shape of your Avatar. Layers at times have to be specific to the Mesh designer base. Heads, unless made custom, don't change so others may have your Avatar's look. Mesh heads and bodies offers more realistic options. Non Mesh heads have a cartoon quality. Avatar cannot walk and move without a movement (walking, dance or posing) nod. Avatar is not nude undressed. Nude body and body parts can be restricted and must be purchased. IMVU's age restrictions are 12 and up. Must be 18 + to purchase Access Pass (AP) membership to buy and own adult items standard in SL.
Mesh Heads:
Pro: Skins and makeup can be added. Skins for head and body can be mixed and matched. Mesh eyes and facial trigger poses are available. Mesh hair is available but sold as a single color per style. Hair base not needed.
Con: Neck and body can be brought separate so necks seams show discoloration and at times head skin doesn't match the body skin tone. Makeup and accessories are not universal. "layerable" clothing items can be used, but may alter other items when they overlap. Animated hair is available, but is rare and doesn't offer free movement just an animated loop.