I think both the 40K and the 40K+Rosigma are worth playing because their gameplay is very different.
By example in 40K due to your troops and enemies not being very resilient , the fights are more fast paced, while in Rosigma all armors are stronger and units (both yours and the enemies) can also dodge making fight take longer.
Also in 40K it feels like there's a main enemy in your game, while in Rosigma every single faction is active at the same time, so basically 40K feels focused and Rosigma feels chaotic.
And more... all influencing the actual gameplay, so in the end both mods will give you a different gameplay. I have a preference for Rosigma (along with some minimods for things i don't like) but 40K is good in itself.