I hate the term "bondo" everything is bondo. One for sure thing don't use anything made by bondo. There are many types of filler out. Lightweight plastic filler, fiberglass filler, fiber plastic mix, metal 2 metal which has a high content of aluminum. I've used all of them under the brand name Evercoat. What you want to use depends on what kind of job your doing. Different filler types have their pros and cons. I mostly use the fiberglass and the lightweight plastic filler. Sometimes alone sometimes together. Depends on you, what you are doing and how much effort you intend to exert. I recomend anything by evercoat, they make good products.
hey ya i like that evercoat, esp one that comes in red can and its yellow in color, it finishes off very nice in my opinion, i dont like blue filler made by them though... and i used metal on metal, good stuff, i didnt test if it would corrode though, but i guess it would be better on restoration for like my bel air than regular filler, more strength..