Folks often ask "what is the best SCM tool" or "is there a comparison of SCM tools available." I would say that generic comparisons of SCM tools are sort of like generic comparisons of cars. Somebody looking for something that is fast and doesn't care about gas mileage probably wouldn't think much of a Prius.
Currently, IMHO, the SCM market is not a commodity market where the products are generally about the same. There is a wide range in both pricing and functionality. So, you really have to know your requirements and the priority of those requirements. A good initial priority to figure out is: which is more important to the organization; price or return on investment? There's no sense in looking at all of the tools on the market if there's not enough budget.
Generally, a good way to acquire a new SCM tool is to start from a well defined set of requirements, create a shortlist of 3-5 candidates that seem close, take a closer look and narrow that down to 2, and then do a proof of concept on those 2 and pick the 1 that most closely meets your requirements.
Depending on the size of the development group and the size of the company, acquiring a new SCM tool can be an eye-opening experience. Finding a tool that meets your requirements is perhaps the easiest part of the process. Some of the harder parts are: figuring out who all of the stakeholders are, getting all stakeholders to consensus on the requirements, getting upper-management buy-in, and securing the budget for the purchase. Getting budget and requirements to match is generally the hardest part and typically involves writing some sort of business case to justify the purchase.
Here are some typical stakeholders: CEO, CFO, Purchasing Agent, Legal, VP of Engineering, Director of QA, key developers, and Release Engineering. Some of these may seem to have nothing to do with SCM, but they do get involved in large purchases that affect the whole company. It is better to make contact with all of these folks early to get the full scope of what is required to change SCM tools so as not to get surprised later and potentially impact release schedules.
If you make a recommendation that fits the budget and best meets the other requirements, but there is still a big gap in the requirements, don't despair! If you've involved all of the stakeholders and built consensus, you have a good chance of leveraging that gap to increase the budget.