Have you ever built a computer before?
You can have a lot of fun building your own box, and you can learn a lot.
But sometimes "learning experiences" can be costly.
Buying a Dell will give you a much easier warranty path if something goes wrong. If something goes wrong with your DIY box you may have a tougher time with warranty claims.
If you do decide to build your own, just stay away from the bleeding edge, stick with main stream brand name components and you may avoid issues.