Which one's better? And why?
I know that citruses are mass produced by grafting, so that would mean grafting has some advantage, m?

For container trees the cons really don't stack up as bad.

Agreed with David re. containerized trees.

For field-grown trees, there are several reasons to graft:

1. Some scion varieties, especially sweet oranges, have very shallow root systems, so are susceptible to drought. Frequent irrigation is necessary. Many rootstocks have much deeper root systems.

2. All varieties of orange, grapefruit, lemon, and lime, and many varieties of mandarin, are susceptible to phytophthora root rot (Phytophthora citrophthora in most of the world; P. nicotianae pv. citri in Florida). Many of the popular rootstocks are substantially resistant.

3. A rootstock can often improve fruit size and/or quality, or overall productivity per acre. In some cases (grapefruit especially), it can also improve the flavor quality of the fruit.

4. For cold-susceptible varieties, a rootstock may provide several degrees of additional hardiness, compared to own-root.

5. Plants grown from cuttings or air layers tend to produce their roots at approximately a 90° angle to the stem, then curve downward. In a hurricane or other major wind storm, that sharp angle is susceptible to breaking off when exposed to torque from wind on the branches above, so the trees may be ripped out of the ground, rootless. In 1992, Hurricane Andrew did just that to most of the own-root 'Tahiti' lime trees in Dade County Florida. Grafted plants are usually on a seedling rootstock, and so tend to have a more vertically oriented taproot, which is far more resistant to wind breakage.

6. Some rootstocks are more tolerant of very high or very low soil pH, salt, flooding conditions, drought conditions, nematodes, or other problems in the soil, that an own-rooted tree may not tolerate. The fact that you get to choose the root system, rather than just taking what you get with a cutting, is, in itself, a potentially great advantage.

But in a potted plant, where it won't experience drought, flooding, hurricanes, etc., and where you the grower can control soil pH and other factors, an own-root plant may be just fine.

But why nurseries that grow containerized citruses (that are not supposed to be planted into the ground) still use grafting? Wouldn't it be easier for them to just root cuttings?

If you're good at grafting, you might consider it "easier" than cuttings. But at least in the US, most nurseries that produce citrus trees produce them primarily for the commercial industry, with potted plants for individuals strictly a side-line to sell a few extra plants. So it makes sense for them to produce a tree that would be good in either situation.

Meyer Lemon is usually grown from cuttings in New Zealand for planting in the ground. Cutting grown Meyers make lower bushes and are mass produced for 6 inch pot sales. I see one wholesaler is growing Persian Lime the same way however usually everything else except Meyer is grafted.

There are also a few varieties that do well on their own roots that are grown from seed--like satsumas.

Another major reason to use Citrus seedling rootstock and budwood is to control viruses. Polyembryonic rootstock seedlings should be virus free, but still highly uniform/ clonal. Virus indexed budwood trees, should be of known virus status. This may be harder to manage if Citrus was mainly cutting propagated, as the number of cutting mother trees required to supply the scale of Citrus propagation in many countries would be far larger than that for the seed/ budwood system.

Has anyone here grown a sweet orange on it's own roots long term? I know sweet oranges have the worst root system, so I'm curious. Don't they also produce some of the best tasting fruit? I ask because I'm planning to root some Cara Cara and Midknight Valencia cuttings.

One of the most beautiful groves in the Lakeland Florida area is sweet orange budded to sweet orange. No blight, hardly any skips in the block.

You are right that SwO roots give high-quality fruit, and the trees are relatively cold-hardy.

The disadvantages, of course, are shallow roots and susceptibility to Phytophthora. I don't know about 'Midknight', but regular 'Valencia' is somewhat less susceptible than other sweet oranges.

Malcolm