They don't contradict one another, it means you will receive an equal amount of *insert whatever here* for an equal amount of labor. Not like how things work in capitalism, for example in capitalism one works 40 hours for a weeks work yet he only receives a fraction of his labor value as a wage. The employer keeps the other fraction as profit, this is called surplus value...
http://en.wikipedia.org/wiki/Surplus_value.
The choice between socialism and capitalism is an easy one, either you want the system to be for and controlled by, and benefit, the majority or you are happy being exploited by the minority.
The way things are now is we, the majority, work, and in working we make the minority, those with the means of production, wealthy off of our labor, we are not receiving the full value from our labor.
In light of all the facts, I suggest you read some of the links I've provided, socialism is an easy choice to make considering we are being robbed of our rightful wages from our labor. Unless you are bourgeois socialism should appeal to everyone providing you have a correct understanding of socialism and not a capitalist propaganda view of it.
A good book to read is the two volume set of MARX & ENGELS "Selected Works" you can find it on abe books for under $20 for the set if you look around.