I rarely draft, but when I do
1) Protective leaders are the best for drafting as they always get the CG and Drill I promotion. A drafted unit comes out with half the experience so a lone barracks get's you only 1 XP. Thus drafting protective gunpowder units in a theocracy can pump out pinch promoted units.
2) If possible draft most of your army from food heavy cities so they can regrow fast. The barracks get's +2 happiness which compensates for the +3 unhappy for the draft. Also the lower civic upkeep of nationhood allows you to sacrifice the science slider to 10% culture. Also if you are still in HR then there is no net happiness loss prior to adopting nationhood.
3) Before adopting nationhood spam theaters and then build the Globe theater in a food heavy city. You can draft unlimitedly from the GT city as you get no unhappiness but need to regrow population fast.
If done correctly, in 10 turns you can have an overwhelming drafted army (3 per city) of 30 gunpowder units. Not bad if you just got rifling and noone esle has it.
Regarding draftign versus the whip, you lose ALOT more population whipping gunpowder units than drafting them. You just need to plan on dealing with the unhappiness.
One final note, I never really adopt nationhood much (unless I want teh espionage) as the extra commerce from Bur and FS far outweigh the draft, at least in my games.
The best drafting leaders
Saladin: Protective and spiritual allows you to massively draft for 5 turns, then revolt back to FS or Bur. With theocracy running you have just drafted 15 units with 3 promotions, perhaps more if you have a few settled GGs.
Gilgamesh: Protective and creative (for the fast theaters).
Churchill: Protective and charismatic: Drafted redcoats (CGI, Drill I, +25% versus gunpowder) are extremely dangerous. Add theocracy, a barracks and 1 GG and you are only 1 XP away from another promotion and thus added GG points.
Tokugawa: Agressive and PRotective: Combat I, drill I, CG I gunpowder units?? Hell, who the hell even needs a barrakcs! He can also draft some mean Samuri if you get nationhood before gunpowder.
Zara: Creative and Organized: Again fast theaters, but also he has a draftable UU which get's Drill I and II for free.
Honorable notes to the French and Ottoman UUs which have draftable UUs However you are relying on their inherant abilities which are not upgradeable.
But seriously, CHurchill Redcoats are the most fun unit to draft in my opinion.