That's a very astute question! :tup:
A VPS is generally pretty limited in terms of storage, traffic and CPU usage (which is often the most limiting factor - especially if you're using canned apps like WP or Drupal). I have a managed, dedicated server and it has reasonable storage and traffic quotas AND I get to use the CPU up to 100% (which I never expect to get to). The key, here, is that the server's own staff is looking after all the security and daemon updates as I can't afford to spend 24/7/365¼ of my time after learning all the intricate details of how their hardware (which is top notch - WebHostingBuzz.com) is configured out of the box. All it takes is a quick ticket to get their support staff to answer questions or handle the rare problem (WHB has a fantastic support staff).
I would expect that your host would also love to have the maintenance contract with you for your VPS but I'd recommend that you assess your storage/traffic/CPU usage before you spend any time (money) on server upgrades and maintenance.
If you can't quite reach the dedi level (managed, to be sure), then I'd think that a reseller account would be best for you. Their quotas are generally more liberal than a VPS and the host takes care of all the daemon upgrades but you get to manage your own (sub)accounts.
If you have too many (sub)accounts for a host's reseller plan, they're pretty inexpensive so buy another account OR put the storage/traffic/CPU hogs on a shared account. Multiple accounts are no worse than a dedi as I have mine segregated into three accounts and manage each from their own cPanel (or my WHM which manages the accounts).
Lots of options for you but you must assess them against your perceived requirements (and further expansion).