It has nothing whatsoever to do with who has more power or less power. It has everything to do with name/character recognition on a national level and the ability to put together a campaign.
Since there are 100 senators and 435 representatives, it is just naturally easier for a given senator to gain name recognition on a national level than it is for a representative. Also, senators run for office on a state-wide basis, while representatives only have to campaign within their own district. Consequently, senators already have a broader base of support and organization within their own states than do representatives. So if a representative and a senator both decide to run for President, the senator already has an organizational head start and is also more likely to be known and recognized in states geographically distant from his own.
The difference between the two bodies, Maciamo, and it is a difference that not even many Americans seem to recognize anymore, is basically this:
The House of Representatives represents the populace, the people. This is why more populous states have a greater number of representatives. California has somewhere around 50, I believe. A few states have only one representative. (The Constitution guarantees that each state will have at least one).
The Senate represents the states. The states should not be thought of as administrative subdivisions of the national government. Think about it in the "state=nation" sense of "state" and it will make more sense. Each state, regardless of population has two senators.
Legislation can be proposed in either the House or the Senate, but must be passed by both prior to being sent to the President. Each body provides a check against the other in this way. Spending bills may only originate in the House, though. Since legislation can originate in either body and must be passed by both, you can see that viewing them as an Upper House and a Lower House is not really appropriate, at least as far as legislative function is concerned. There being far fewer senators, and they being elected on a statewide basis, there is a bit more social prestige attached to the Senate, though.