Corsair gets into mid-range power supply with this new PSU, but you may as will stick with the RM650.
One of the central truths to the PC industry is that no matter how much focus is put on the high end, the mid to low-range products are the ones that generate the money that keeps companies alive. Corsair has been king of starting out with enthusiast products, and following with more reasonably priced models.
The CS650M is such a product. It's slightly cheaper than the recently released RM650 PSU, by about $20. For this you get a supply with 80 Plus Gold efficiency, with a semi-modular design that should keep you from needless cable tangles, in a reasonably priced build. This is a very competent supply for those with average needs. But given what you get for that extra $20, we'd still suggest the RM650 is ultimately the better choice.