I use…a bit of everything. Tyme2 is my billable hours tracker. It’s flexible: Groups (which I use for different types of work: press, social, investigation, etc); Clients/Projects, Tasks/Subtasks; and you can archive any of those individually. It also allows you to set time or currency-based budgets and tracks your progress.
I then manually move that information over to Xero, which I use for invoicing and balancing my books (lots of pass-through expenses to clients).