Collagen (bovine and human) was a common filler years ago, and was used predominantly for lines, wrinkles, and lip enhancement.
Now that HA (hyaluronic acid) fillers such as Restyland, Perlane, Juvederm Ultra, and others exist, they have virtually replaced collagen in most modern plastic surgical and dermatologic offices and medispas. HA fillers are softer, clear (not white), easier to shape in the tissues, and last about twice as long. They cost a bit more than collagen, but not twice as much, and since they last twice as long--that's a plus!
Regardless of the filler use for lip enlargement, the skill of the injector is the most critical issue, not the exact brand or type of filler. Seek a board-certified, experienced plastic surgeon or dermatologist who uses lots of fillers. If you like the look, and get tired of paying every 6-12 months for more filler, you may wish to consider dermal grafts for your lips. This porcedure uses your own tissue as a living, permanent, soft, natural, and sensate lip enlargement technique, It can be done under local anesthesia, and is very natural and "non-fake!"