We all know the famous debate between these fundamental ideas, and personally I would call myself a secular progressive, but I honestly want to hear arguments for capitalism in the modern age. Most people I talk with cite Adam Smith or early 19th century thinkers.
I suppose my main argument for socialism would be that we are social animals in a colony like setting, so the health of all individuals is directly related to health of all individuals, so competition would be negative in the scenario, and I can point out the power of community, and even almost somewhat communist mentality with the concept of open source software and code. Linux is, in my opinion, the best operating system, and it was created in it's entirety from people wanting to make a decent open source OS.
I can, however, think of markets in which greed can be useful. Many entertainers would not be able to produce the works they do without such funds. Many movies have a hefty budget, but at the same time there are artists that enjoy giving their work away from free for people to enjoy it.
Perhaps more of you out there can bring in more arguments for capitalism/profit motive/self interest.