A person with a lot of knowledge and another who knows how to express himself/herself in an easy and convincing way. Who is a more successful person?

knowledge & expression ability are complimentary! You cannot express without knowledge, and on the other hand knowledge without expression ability is not useful.

The person who knows how to express himself/herself in an easy and convincing way is much better than a knowledged person.

This would depend on the job.
Expressing yourself to 'get ahead' would be required when the job you are in requires 'expressive people'.
A knowledgeable person has the knowledge the expressive person doesn't.
Knowledge is Power.
Power creates self confidence.....
creates expression!!!1

knowledge creates self expression and confidence

Yes a Person who can convince is more sucessful

Its very nice question, I can say, its look like once you differentiate between good & bad teachers, the only difference between them, that the good one can express his ideas in a good way, while the other could not, so at the end, the rewards will run directly to the one which have ability to express his ideas, and this is a vital part of good communications.

The person who knows how to express himself/herself his/her knowledge in an easy and convincing way is a more successful person because if a person don't know how use his knowledge in a proper way then what is the use of his knowledge. It is same like a fruit seller who have got a good variety of fresh fruits in his shop but without knowing how to sell them all. after some time all the freshness of the fruits will go and will get damaged  and he will suffer a great loss.

Convincing power is not enough without meaningful knowledge for success.
At one stage convincing power will not work without knowledge.

Well, for me the person who have enough knowledge. knows too how to speak. because he has got all the materail what he need to speak/tell about. interrelated they are both. and if a good speaker doesnt have material he wont be able to speak.

It all depends on the job requirement. If the requirement is in reearch or technical what matters foremeost is his knowledge and skill while for a marketing job, the ability to put himself across convincingly is what matters. Ideally a combination of knowledge, ability to present himself and has convincing power, then he is a sure shot winner anywhere.

Sales should own convincing skills coupled with some knowledge.