Men of War: Assault Squad features a completely new cooperative skirmish game mode with access to five different nations (Russia, Germany, USA, Commonwealth and, for the first time ever in the Men of War series, Japan) as well as increased realism and accessibility.
Men of War is successful because each game naturally generates its own narrative. It is brilliant because it has character. It is great because (in campaign missions at least) you feel attached to your troops.
Assault Squad is Men of War minus all of this, and is horrid.
Its lone addition is the new Skirmish mode: a depressing, soulless grind against entrenched AI. Instead of wire-tight combat against a fellow human comes wave after wave of artificially buffed but simple-minded robots; in place of the memorable set pieces of authored missions is crushing repetition both within and between maps; and both motivation and opportunity to delve into the game's intricate simulation are lost beneath a constant stream of replacement troops.
Buy and love Men of War, but skip Assault Squad. DigitalMindSoft apparently don't understand what makes their games so good.
(Edit: Assault Squad does also include all MoW multiplayer modes and maps. Those are excellent, but have not significantly changed from last time.)