Use both together. The end game power civics package I use for Domination with a SE is often : Police State, Nationhood, Slavery, State Property, Theocracy. That gives +35% production bonus for Slavery, add in the Kremlin for another 50% boost and you can produce oodles of troops and ignore the war weariness.
Nationhood can save your butt in those oh **** situations(especially with spiritual), while maintaining decent city size, slavery has much more drastic results. Slavery just is not worth it any more at the time when nationhood comes around, troops just cost too much.
Sorry, this is just not true. Police State, State Property, and a forge gives +60% for troop production (without considering factories and power) and the military bonusses for HE and MA and drydocks all help Slavery and not Nationhood. But the really huge boost is the Kremlin which makes Slavery 50% more effective before the other bonusses apply. Then there is the difference in unit costs on marathon versus normal. On marathon most things are multiplied by a factor of 3, research requires 3 times as many beakers, building cost 3 times as many hammers, slavery gives 3 times the hammers per pop, 3 times as much food is required to grow a city but the big difference is in unit costs which is only 2 times the hammers of normal. For instance a rifleman costs 110 hammers on normal and 220 on marathon, and the cost for drafting is the same 1 pop on both. This makes drafting a lot less desireable on marathon and when all the above factors are taken into account Slavery is very competative.
Here is a worked example for marathon. A drafted rifleman costs 1 pop. A rifleman costs 220 hammers and 1 whipped pop gives a basic 90 hammers on marathon, so without any modifiers it takes 3 pop to give 1 rifleman plus 50 hammers overflow. But that is very misleading since it almost never happens that way if you're geared up for war. The Kremlin increases the basic hammers by 50% to 135. Then we have the basic production modifiers (PS, SP and forge) which increases it by 60% in all cities, that's 135 x 1.6 = 216 hammers (which is almost the same). If we then include HE and a MA we get another +150% for military units or 135 x 3.1 = 418 hammers which is nearly 2 riflemen for 1 pop. In this extreme case Slavery is twice as efficient as drafting
And that is without considering factories or power which I find give too much unhealth to be worthwhile. The Kremlin is essential if you want to use late game Slavery and I've found in my last two games that I head through Scientific Method and straight to Communism not for State Property (excellent though that is) nor for the free GSpy but for the wonder. That gives a +50% boost to my food to hammer conversion rate and is massively empowering for a high food SE. It's arguable that the Kremlin is better for a SE with Slavery than it is for a CE running US.
Nationhood gives +2 happiness with barracks (already mentioned) and also +25% EPs which is useful if you've a big empire with a lot of jails, intelligences agencies, security bureaus and run spy specialists. If you're going for a late game domination then the combination of powerful military production, immunity from war weariness and overwhelming espionage is a strong strategy. The Nationhood and Slavery civics fit very well into that strategy.