Which is the best mode to use?

I've been reading that if you use D3D, the horizon looks incorrect?

I've tested both on my 3070, and the horizon looks the same.

Is there any advantage to using one or the other?

I tested it on a 10 year old laptop the other day, and Glide (through nGlide wrapper) gave good frame-rate, whereas Direct3D was too slow.

If you use dgVoodoo it's probably better to use D3D as you can crank anisotropic filtering up to 16 and usually have sharper graphics.

It's my general knowlegde though, I haven't specifically tested it with this game.

I have numerous versions of EAW which run in Win 7 or Win 10 depending on which drive I use.
When I buy a PC I make sure that I have an AMD graphics card. Ages ago nvidia cards supported horizon fog in d3d, but AMD cards did not., and needed glide. However the situation was reversed some time ago. Currently PCs with nvidia cards do not support horizon fog in d3d, and glide is needed.
There is a mountain of mods available for EAW all made by amateur enthusiasts since 2000, but some of the available skins are not glide compatible.

Jel