You just can’t use your points to stay anywhere but the DVC Resorts (like you couldn’t book a Disney cruise or trade out through Disney to their “Concierge Collection” and the like… but that’s not to say that you can’t trade your DVC points with someone else’s RCI points through a Timeshare Trade forum). However, the staying at the DVC Resorts is really the best use of points, anyway. And if that’s all that you want to do, Resale might be a good option for you.
The other downside to resale is that it takes a little bit longer to acquire your ownership (more steps to go through… counter offer, owner accepts, Disney agrees, etc… it’s a little bit more work, but very well may be worth it to you), and the expiration date is sooner than it would be if you purchased brand new. You may or may not be able to use all of your points the year you purchase, depending on your contract (but on the plus side, you could have previously banked points ready for you to use).
Those are the only differences I know through research. I’m sure longtime owners like rlcarmichael who I believe has purchased both directly through Disney and through resale will have more information to share.