There’s quite a few differences.
- Who you pay: in traditional feudal taxes and rent, you’re paying to the lord of the fief, who is bound by recriprocal feudal obligations, the custom of noblesse oblige, and a certain long-term interest in keeping his tenants a going concern. With tax–farming, you’re dealing with a middleman who is bound by nothing and has no long-term interests at all, since tax-farming contracts are usually strictly time-limited.
- How much you pay: traditional feudal taxes and rents were fixed by custom and law, and usually recorded as well. While this had the effect of making past injustices very hard to shift, it also meant stability and predictability. Tax-farmers, by contrast, paid for their contract through an auction process, were looking to turn a profit on their investment, and could impose whatever rates they wanted to – which meant higher rates and substantial unpredictability.
- What you pay: a lot of feudal taxes and rents were either in kind or in the form of labor, which is handy when you’re a farmer who has lots of agricultural produce but not much cash on hand. Tax-farmers are far less interested in going through the messy process of storing and selling these goods, and tend to want cash on hand, which is not good for farmers.
There’s also an issue of sub-infeudation – i.e, the layering of land ownership from king down to lord to lesser lord to knight to peasant. So normally in the Crownlands you would pay your rent and your taxes to your local knight or lord, and then they’d pay their feudal taxes to their liege lord, who pays their taxes to the lord of the principle house, who pays their taxes to the king.
If you lived on the King’s own land as opposed to lands that he gives out as fiefs to the various lords of the Crownlands, you have a simplified system where you pay your taxes and rents directly to the King as your liege lord. Unless the king sells the right to collect to some tax farmer…