Long answer:
Some history first. R.E.D. stands for Regiment and Ethnic Diversity, which refers back to the previous Civ4 incarnation of this mod ("Regiments, Formations, Air Force, and Ethnically Diverse Units") which started out by combining several Civ4 mods, including Ethnically Diverse Units, and Regiments.

These 3 mods all use basically the same set of available units. The mod creators don't make the models; they just compile the stuff that other talented people have shared.

Ethnic Units was based on an old version of R.E.D. Modpack (v.11 if memory serves), before Gedemon switched to the SQL table format it uses now. Ethnic Units removed the formation and smaller scale that R.E.D. always included. For a while it included more units, partly because Gedemon had stopped updating for a time, but also because he preferred not to (arguably) shoehorn in a unit model that was available but didn't quite fit. Gedemon eventually did make some updates that closed the gap considerably. I took over for a little while and my v.28 semi-official update eclipsed Ethnic Units by a wide margin since BrutalSamurai22 hasn't updated E.U. in years.

Ethnic Diversity is a fork that I originally introduced during one of Gedemon's absences, prior to Ethnic Units, but I discontinued it when Gedemon came back. I reintroduced it again in 2016, adding even more units than my unofficial R.E.D. v.28. It uses vanilla scale only at the moment, but R.E.D. scale has been planned to be added as a separate addon (though I've not had time to finish).

Short answer:
1. Unlike R.E.D. this uses the "vanilla" scale and formations of the base game.
2. Lots more units!

Number of unit entries (though some units are shared by multiple civs)
Ethnic Units (v.31): 576
R.E.D. Modpack (v.27): 831
Ethnic Diversity (v.29): 1623

Number of unit models (though some units include multiple models)
Ethnic Units (v.31): 619
R.E.D. Modpack (v.27): 771
Ethnic Diversity (v.29): 1041