I find that the two products serve different needs.
We use CommitCRM to manage service requests, assign jobs to technicians, track hardware and software licenses, dispatch jobs to technicians and schedule online visits and to log time and parts. We also use the web portal when on site. Our customers also put in new service tickets and see the status of existing ones.
BTW, We also sell time blocks service contracts and track them in CommitCRM.
We use QuickBooks to handle our accounting - handle payments, print invoices, manage and track our bank account, generate profit/loss reports etc.
We use the link between CommitCRM and QuickBooks to create QuickBooks invoices directly from CommitCRM.