Hello, I am about halfway through my driving school and have two job offers, one is working for a company getting paid cpm and the other is by percentages.

NTB is offering .28 cpm for first year then .31 cpm after a year it is mostly dry vans and reffers

Green lines halls metals on a flat bed but pays 23% on 100% on rate
(Not totally sure what that means)

Any advice from drivers would be appreciated, thank you.

Mileage, you get paid by the mile.... when you look at this, try to find out if it's paid by computer miles, route miles, or hub miles. There is a difference. Loaded or empty, you'll be paid X amount per mile.
Percentage, you get paid a percentage of what the load pays to the truck. If you work for someone who keeps deadhead (empty) miles to a minimum, and avoids hauling cheap freight, you can do alright. But you get a fixed amount per load, and you don't get paid for running empty. Also, when looking at employers who pay percentage, find out if that percentage is just on the line haul, or if it's on the gross amount paid to the vehicle. I worked for a guy who paid percentage based on gross to the truck.... which meant that was I was paid was based on line haul, fuel surcharge, and other allowances paid to the truck for that load. It also meant that I only got a percentage of tarp pay if it was included in what was paid to the truck. Tarping wasn't too common of a thing for me to do, so that didn't bother me. For someone who runs 100% tarp freight, it might.