The simple, core difference...

The socialist argues in favor of one person or group of people (the government) spending the money of someone else (the taxpayer).

The capitalist argues in favor of the individual spending his own money as he sees fit.

Are you more likely to make foolish investments with your money or with someone else's money? Certainly it can be said that a person would be more likely to evaluate risk and reward when considering investing his own money than he would be were he to evaluate investing someone else's money, and therefore the individual investing his own money would more often than not make wiser choices than the government agent who has no personal stake in the outcome of the investment.