I beg your pardon? SP2 more stable? Boy was that a laugh. Interface better? Ha, good joke old boy.
In sheer fun terms, Superpower 2 wins hands down, as it funny when the UK elect a Sien fein government, Taiwan leeps to the defence of Israel when Bolivia declares war on them , and when the United States nukes cuba , so it must be a fun loving AI.
Fist of all, SP2 and HoI2 can't even be compared, they are compeltely different games in both style, grace (or in Sp2's lack there of) and technical aspects. HoI2 kicks SP2 in every direction under the sun.