While the difference for clients mostly comes down to cosmetics and organization, for the host, different control panels offer a variety of tools and features.
cPanel offers the obvious benefit of being able to say you provide the most popular control panel.
Nodeworx, the admin side of Interworx, offers advanced permissions management, tools for handling clustering, branding capabilities for resellers, and a number of other features to make a host’s life easier.
Plesk gives more capabilities to clients, which can reduce the administrator’s workload. It also structures its databases around customers as opposed to domains, making it easier to set up reseller accounts.
There is also the issue of cost, which will carry over to your hosting accounts. Some control panels, particularly open source or home-built solutions, can offer substantial savings, but you will need to weigh this against offering a control panel that clients are familiar and comfortable with.