Mild steel will be fine if no food will come into contact and would/should be cheaper, however stainless would last alot longer.
If you decide to with stainless steel and its going to be sat on the coals and get extremely hot then try to get a "cheap" grade of stainless steel to reduce warping. If it needs to be food safe make sure its 304 grade stainless steel (316 is even safer but much more expensive)
Hope this helps👍
Out of curiosity, what size and thickness would you require?