Haven't tried SOTS but heard there were major issues with the second part during release so steered away from it
I have played Hearts of Iron II though and if they make this game half as good, it will be a serious competition for other space strategy games
You can see already that they have put real effort in identifying the weak spots of the 4X genre and aim to address them right at release date
Some of their approaches remind me on solutions used in Endless Legend which also set out to do the same