I've never really experimented with matrices much, but from what I know about them I wouldn't recommend them - they're more easily customisable in terms of adjusting height, I guess, but you can't apply shaders or multiple layers of texture to them. I've read a few people saying they're annoyingly slow, as well (the matrices, not the people). I reckon it'd be almost impossible to code LOD functions for matrices too, because once you've generated a matrix, you can't change the number of tiles it has, or move them in any way except vertically :/
The advanced terrain commands are also quite out of date, but they might be a good idea for starting out with terrain! You need to have a heightmap for those, as you probably already know, but they pretty much handle themselves, which is great.
For your actual open-world project, I definitely wouldn't recommend either matrices or advanced terrain! I'd say your best option would be to check out the awesome Advanced Lighting code written by Evolved, which has examples for creating very detailed terrain. Here's the link:
http://www.evolved-software.com/advancedlighting/advancedlighting
Figuring out how to use Advanced Lighting can take some time though, and if you're new to DBPro, it'd definitely be jumping in the deep end!
Good luck with the project!
