what i can say about gravel and rock is that if you ever get a leak in it, well youll be rolling the dice on finding the leak. torch down is good and can be coated, after a couple years, with elastimeric roof paint.
For what it's worth, EPDM rubber was developed as an improvement over tar and gravel for flat roofs. I've had EPDM on my flat rear shed dormer for 20 years and it has done a good job. No problems at all. It is also very easily repaired if it gets damaged by falling branches , etc. Carlisle is also a good manufacturer of EPDM. If it's my money on those roofs, EPDM w/out a doubt.