In the past few days I've seen several discussions (based, so far as I can tell, much more on politics than any real science) about how the new NASA Space Launch System (crewed spaceflight to the moon, Mars, asteroids and beyond) should be abandoned, either in favor of the old Constellation/Ares program or Elon Musk's SpaceX program. Some of the arguments put forward have been patently absurd, so I made a quick graphic here to illustrate why.
So, the Ares I (Bush administration) program was abandoned because Ares I was grossly over budget, years behind schedule and had absolutely nothing to show for any of this. The very large R&D budget had all been spent on the R, with nothing left over for D. The later-to-be-developed Ares IV and Ares V rockets showed more promise, but were scrubbed along with the Ares I.
The SLS (Obama administration) Block I, like Ares IV and V, is based partly on "legacy" Space Shuttle hardware. But in order to meet the congressional mandate of having this off the launchpad by 2017, NASA heavily relied also on legacy hardware from the Saturn V. No new hardware means no R&D; SLS Block I is, amazingly, actually a bit ahead of schedule. The SLS Block II spacecraft, which rely in turn on SLS Block I technology, are essentially the Ares IV and Ares V. See illustration for comparison.
Arguing that Ares is a better platform than SLS is akin to arguing that Muhammed Ali was a better boxer than Cassius Clay.
SpaceX, meanwhile, has been fantastically successful so far at launching uncrewed cargo vessels to the International Space Station in low earth orbit, and by 2017 the Dragon Rider spacecraft will be taking crews to the ISS as well. But the Dragon program is basically a redux of the old NASA Gemini program. Which is excellent, and inspired even, but the transition from Gemini to Apollo was one of the most expensive undertakings in human history. Musk could sell a thousand Teslas a day and not be able to build something capable of taking humans to Mars. It is not possible to simply "scale-up" from an earth-orbital spacecraft to an interplanetary (or even translunar) one; the Soviets tried this with disastrous results. Musk has discussed the possibility of developing a rocket to Mars, and I believe he has the know-how to do it. But it's not on the drawing boards yet, it doesn't even exist as a power-point presentation, and I have yet to hear a plausible answer to the question of how he would pay for it.
So, for better or worse, SLS and the Orion spacecraft are America's space program right now, and there isn't a credible replacement for them in the foreseeable future.