The problem with choosing a source control and issue-tracking system isn’t so much finding one that’s right for you as picking through the huge range available. So, what might sway your decision? Some things (not an exhaustive list) to consider include the following:
Open source or commercial?
Do you need to host it yourself (behind your firewall, for example), or do you want to use one of the many services that provide hosting for you?
Do you need your source control and issue-tracking systems to be tightly integrated with each other?
What level of support for distributed development do you need?
I can’t possibly give a complete survey of all the different source control and issue-tracking ...