I'm pretty sure i was the first one... haha.
Chopping is basically free hammers. The opportunity cost for chopping is very low (you lose a few worker turns). Its mostly limited by the number of forests.
slavery, on the other hand, is an alternative to just simply working tiles or specialists. In many cases you'll do better by not whipping and just working tiles. So even though you whip 100 times, considering the opportunity cost of whipping is just working tiles--you still probably don't get as much benefit out of the ability to whip than you do out of the ability to chop.
Generally speaking -- slavery with a granary produces about as many hammers per turn as just simply working plains hill mines with the extra citizens instead of whipping them every 10 turns.
The real advantage of slavery is not hammers, but the ability to hurry production -- sometimes you just want things fast. So with total production being equal (plains hill mines vs whipping) -- slavery still wins out due to the fact that its production is frontloaded not backloaded. However, the production hurry benefit is only relevant if your cities are already grown out and already have granaries. If you had to grow your cities out just to whip them, and also probably whip a granary too, then you aren't getting any hurry advantage.
The other advantage of slavery is that you can convert food into hammers even if you have excess food and no hammers. This turns cities that have no production but lots of food into extremely productive cities. Generally speaking, 1 food is worth about the same as 2 hammers -- this is true even if you're not using slavery--up until you reach the happy cap, assuming you have adequate tiles to work with the extra citizens (1f2h tiles or 4h tiles, for example -- 2 extra food will allow you to work one extra 4h tile or two extra 1f2h tiles, which is why food is worth about twice as much as hammers.). After the happy cap, each food is still worth about 2 hammers if you're running slavery (since you can whip unhappy citizens at about this efficiency). If you're not running slavery, at happy cap excess food is useless past what your citizens need to eat.
But once you start whipping away good tiles, slavery is often a really bad deal. If you can work good tiles with all of your citizens (once that produce 5+ total f/h/c ), its rarely a good idea to whip except in warfare situations. So, for example, if the plains hill mine i was talking about earlier was on a river, you get an extra commerce out of it, and its probably better to work it rather than whip, unless the thing you're whipping is needed urgently. And if you need commerce, whipping away cottage workers or even lake workers is usually a terrible idea.
If you play as a total peacenik, you can get by pretty efficiently without slavery. proper use of caste system is not that much different -- you may lose a few turns here and there due to having to slow build things you shouldn't, and cities that lack hammers will be pretty hopeless... but eventually you'll be able to build workshops in cities that have no hills, and the extra specialists are nice too.
where slavery really shines is warfare -- you can whip an army as soon as you get the required techs, and you can turn captured citizens into military units very quickly to continue your push. Its really hard to do war properly without slavery.
"We cannot forget your cruel oppression!").
