There is a balance of power split between the executive (president), legislative (congress) and the judicial (courts). When congress is mostly composed of one party and the president is of the other party, things don't happen very easily. Congress can make all the laws they want, but unless they have a large majority, they cannot get around a presidential veto.

So, to answer your question: both are important.

And yes, although I am Canadian, I also hold US citizenship and spent the first 3 decades of my life in the US *smile*

According to the constitution, the President, Congress, and the Supreme Court SHOULD have equal power

They should all have equal power.

Symbolically it's the president, since he* is the 'mover and shaker' of the country because he sets the 'ideological agenda' of how both parties will react to how he acts to their actions. The role of the president waned in importance after the founding fathers died off (Lincoln-Grant being an exception), and only starting rising to the new prominence with Teddy Roosevelt because he punched the public consciousness in the face. Historically senators have had the most power and clout because there's less of them, they do the treaties, and they have longer terms with no limits. But on the other hand Congressmen take care of the finance which is the hot button issue.
http://en.wikipedia.org/wiki/United....._to_the_Senate

*a he, so far...

President, because aside from what the other folks have said (1) I haven't looked at the numbers but I think it is unlikely that the House would flip to the other party and (2) we already had the two years at the start of the presidency where Congress was controlled by his party. There is also the possibility that one or two Supreme Court justices may decide to retire within the next four years, and the president is who nominates the justices for that. The court could change its liberal / conservative makeup for the next 15 years based on this.

The equal powers thing is in theory rather than in practice. They all hold different things that one can do and the other can't. Our system was designed on the idea that congress decides who we go to war with but that changed so everything foreign policy related is de facto up to the president. That's pretty damn important. Other than that though the president can do a few neat things with executive orders and appoint supreme court justices. As important as that is though the make up almost never changes because the judges mostly step down under a president of their party. If Mitt becomes president the democratic judges will wait and the same is true of the republicans. Finishing that life term in the supreme court is really, really rare these days so it isn't as important as people think because we haven't had a streak in a long time where a party held the white house for more than 3 terms (Heck, we haven't had more than 2 since Bush followed Reagan for a term in the 80s). Mostly the president just holds the ability to veto congress and their voice historically held a lot of weight with the people and congressmen of both parties. That really doesn't hold the power it used to though because the country and congress are so divided and uncivil at this point. The presidency does not hold the same power domestically that it used to. As an avowed democrat I'd trade control of the house for a Mitt Romney presidency in a heart beat but that outcome would be straight up impossible based on any polling evidence. The house creates actual legislation and congress can theoretically strong arm the other branches of government in various ways if they actually get behind something. Supreme court struck something down? Find a way to do what you were trying to do legally. President wants to go to war and you don't? Don't fund it. If congress doesn't like something they just have to close the purse strings. If the democrats held the senate and the house they basically run the country unless Romney vetoes stuff but that really pisses Americans off if the president doesn't use it sparingly. If you want to go down in your reelection bid just start vetoing everything congress does and grind the country to a halt (unless those acts are that wildly unpopular).

Long story short, the presidency is powerful but very overrated.