Meh, slaves cost too much all around, you have to buy stuff to keep them in line, manhandle them about, and whip them when they slack off.
Indentured servants are better. The two major differences in slave/indentured is you have to maintain an indentured and set them free after their 7 year contract is up, which is probably why this system was supported over slavery. However since we all know a well rested & fed slave means more moral which yields several times better productivity the first is a given. Infact since the indentured sevart choose to put him or her self in that position you can expect a higher level of moral to start with. And the limited contract is negligible given that you're out adventuring in the world and they will be several levels below you thus likely to be killed before their term is even close to being up.
Other benefits to choosing them over slaves is government support and of course their willful choice to sell them selves into that position. Even in regions where slavery is abolished indenturing would liking be supported which means the local guards and guilds would become part of your restraint system. Given that they choose to be there in the first place escapes are only likely if you grossly violate the purpose of becoming indentured, such as feeding them to dragons for sport. An interesting idea is to take the idea from the indentured and apply it to a villein (a type of serf). Then you get pun points.
Like slaves you can buy, sell, and trade them around. Physically abuse them, which can include rape which offers a special bonus of increasing the duration of the contract should they become pregnant regardless of a lost childbirth. An interesting note is that can be used for a low level campaign hook; an aristocrat is accused of impregnating and forcing abortions to force a no freedom effect, and the local priest is paid off with access rights to claim the aristocrat's house is cursed.
Serfs are another choice, though you would have to be a land owner to own one and it's even worse than slavery. Serfs are bound to the land they came with and are almost impossible to sell off. The biggest problem in owning your own collection of serfs is of course the fact you are a lord at that point. Thus a likely target of other lords wishing to increase their domain. I highly suggest buying a slave over seeking serfs and indentured over them all.