If the title of my post was a bit confusing, I'm sorry. What I'm wondering is if it's better to use a pre-built engine say Ogre3D or CrystalSpace, or if I'm better off writing my own rendering engine? Your thoughts are welcome! :-)

Depends, do you want to spend your time learning to write an engine, or making a game*? The choice is yours, and both are equally valid. Choose whichever you think that you'd get the most out of.

* Note that you'll of course have to learn the engine, but that's less work that learning a graphics API and then writing an engine.

I'd say if you have to ask then probably just use a pre-built.

Sounds more like you're after making a game than building a graphics engine so you might as well save some time since that is a very consuming part of any 3d game.

Well if you had to build the engine from scratch I think I would learn more, and I would know how my engine works and how I could easily make changes. Because learning to use another engine I would think would take almost the same amount of time if it is similar to ogres extensive code base and functions. That's my opinion.

To learn to USE Ogre (or anything of similar size) you do not need to know the working of the whole code base. That's where you'd save time by using a pre-built engine.

But, you're right, you'll learn a lot more by building your own.
So, as Ezbez said, decide which of the following sentences describes your feelings best:
1. I want to learn everything about 3D-programming and have a strong interest in linear algebra and the likes.
2. I have this cool idea for a game and want to make it.

If you chose 1, build your own engine and learn tons about how engines work and why they work that way.
If you chose 2, make a game with a pre-built engine. It takes a long while to build your own engine, and you get little back game-wise at the start.