It all boils down to the amount of separation you have with the other users on the server. With a VPS, you get a little more separation from the other users (normally through a kernel hack, such as contexts or jails, basically super-chroot) compared to reseller access.

Usually you can see (but not modify) which processes other users are running on reseller servers. With a VPS you don't see anything about the other users in the same server, and you get full control of your little area. This can be a good thing or bad thing, depending on how the server is managed.