There are clearly benefits of each, if you choose a big company, you will learn more knowledge and experience. On the contrary, a small one would give you more chances to promot.
This part contains an error involving the use of the phrase "on the contrary." You should only use the phrase when directly contradicting what has just been said. So, you could say
Some say that, if you choose to work for a big company, you will gain more knowledge and experience. On the contrary, working for a small company can actually teach you a lot more than you would ever learn with a large firm.
I don't know if that's what you mean, though. You might just want to go with "on the other hand," like this:
"There are clearly benefits to each. If you choose to work for a big company, you will gain more knowledge and experience. On the other hand, if you choose to work for a small firm, you will have a greater chance of being promoted."
If you go with this construction, note the parallel structure between sentences.