Well it all comes down to versatility along with budget. In a reseller account you have restricted options for reselling, you can only sell plans under certain restraints. When you purchase a VPS or Dedicated server you can customize it to how you feel it would best fit the customer needs and requirements. You can have the packages how you want, with the limits you want. You also get the chance to play with configurations before you actually place customers on the server/VPS. A lot of hosts have lost customers from moving from a reseller package to a VPS or Dedicated server due to trying to configure the server, having it go offline frequently while customers are housed on it. You do not want to be testing for example firewall settings on a box, locking yourself and everyone out then end up waiting an hour plus paying a fee for a datacenter to log in single user mode to disable it while housing customers on the server. Especially when that mistake can be made more than once.
Also, make sure to do proper calculations and try to keep from overselling to badly. Just because a dedicated server is on an unmetered plan doesn't mean unmetered bandwidth should apply to all customers, there would always be some customer abusing the service and ruining hosting for everyone, also another reason to go with VPS/ Dedicated over a reseller. You have control on if a customer gets suspended over the main host suspending your reseller account or customer accounts because the accounts under you would be spamming or using to many resources. There are a lot of benefits to using a VPS or Dedicated. Of course though you would have to know how to run the VPS and Dedicated unless you purchased a managed service. Spamming is a bad thing so it would be a good idea to keep an eye/remove customers who attempt it rather than get a shared or dedicated IP blacklisted. Most VPS hosts will stop the VPS if the IP gets blacklisted and notify you after, or firewall it off so pretty much all services with the exception of ssh is blocked, data centers may notify you to fix the problem then if it goes on for longer than a day or so pull the server.