I would include myself in the both category as well. I typically watch replays in 2D mode.
For my turn, I use 3D for combat because I find it much easier to pick up terrain features in 3D. I do use 2D mode for long range movement, some artillery/airstrike plotting, and to get an long-range overall view of the situation for planning (I prefer that to the 3D zoom out views where the units are harder to see) or LOS checks (I find the in/out of LOS shading is sometimes easier to pick out in 2d mode, but that depends on the map and the terrain).
Regarding the NATO symbols, there is a good chart of all the symbols used in the game in the appendices. Or at least there use to be in the Talonsoft paper manuals. I suspect they are still there in the Matrix pdf manuals, though. The most important things to remember are diagonal slashes for infantry (one slash light infantry/recon/ cavalry, two slashes regular infantry), open ovals for armor, E for engineer, inverted chevrons are generally anti-tank and dots for artillery (bigger dot, bigger gun tubes). Also white symbols are transport.
If you remember those, combinations become a little easier to decipher. So a box with two diagonal slashes and an open oval is an armored infantry unit. A unit with a inverted chevron and an oval is an armored AT unit.
One other advantage to getting to know the NATO map symbols is that it makes reviewing combat maps in books and on websites much more informative (assuming the book is using NATO/WWII Allied symbology). For this, you need to know the unit size modifiers, i.e. the vertical lines, dots and X's that denote sizes from squads up to army groups.
And heaven help you if you have a map that uses the German WWII symbols. I can never seem to remember those.