Yes that is correct, many would argue that humans are the most successful species. Although there is not one measure of success when it comes to evolution. It is very likely that evolutionary pressures that led to the development of humans were very different that then the ones that led to dogs. Unfortunately this process was not documented so have to do with speculations. 

Some advantages of walking on two legs are:
Energy efficiency
Freeing of the arms to carry food back to the tribe

Some disadvantages of walking on two legs are:
Higher strain on the body. Think of back pain and joint pain. 

So we can say that humans walk on two legs because of the advantages and dogs walk on four legs because of the disadvantages. It is hard to really compare. Bipedalism is uncommon in mammals and it could just be that evolution doesn't call for it during the development of dogs. As long as a species can find a niche in which it can survive it is "successful".
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