Collagen was one of the first fillers on the market many years ago.  They are not as popular today since they do not last long.  Generally, collagen injections last around 3 months compared with  Hyaluronic fillers like Restylane which typically last  5-7  months(or more).

Also, the Hyaluronic fillers (Restylane, Juvederm, Perlane) are more fluid like substances which tend to attract water to the tissues for a softer look.  They are well tolerated and have minimal down time as far as recovery is concerned.