Stainless steel or Black steel

As an engineer with some background in properties of metals, the nickel in stainless won't be affected by grilling temps. That said, there are many grades of stainless and some stainless grades can rust. Nothing like plain steel though.

I'd go for the stainless grills.

Go with stainless. Many high end grills come with 3/8" round barstock. My Komodo Kamado comes with 3/8" upper grate. As you can see, the 3/8" rods are only supported around the circumference of the 23" grate. Depending on the dimensions of your grate, you may not need additional supporting rods running down the center of your grate. That will make for easier cleanup, and I can't overstate the importance of easy cleanup.

You should be aware of grill scrapers made for oversized bar stock. The Billy Bar and the Grill Floss are two that will work very well. You must ask Billy Bar to make you an oversized unit for 3/8" because their standard one is for 1/4" stock. They'll do it, though.

Whatevercheap

I use mild steel in my grill grate builds for a couple reasons:

Price - Stainless is much more expensive. I just paid $510 for a sheet of 1/8" stainless and $128 for the same in mild steel.

Non-stick - Mild steel, when properly seasoned is a perfect non-stick surface. stainless is less effective at making nonstick without some sort of coating.

Mild steel is easier to work with. Stainless looks cool though.

Stainless for the weather and coolness factor.

If seasoned properly, your carbon steel grates will last forever.