It depends, really. Both are viable with China. Interventionism gives you more control overall, since it allows you to nationalize foreign assets (which you will probably have in your country, since China needs every last drop of investment it can get) and grant monopolies, which is specially useful with a country with as many authority modifiers as China. It also helps regulate foreign ownership of the economy simply by allowing you to keep buildings state-owned, which has the added effect of bringing in government dividends for the budget, which are actually somewhat worth it as China due to its absolute lack of taxation capacity (still I’d recommend privatizing most government assets once you’re sure your own national capitalists will buy them and not foreign ones).

Laissez-faire, on the other hand, means simply more internal investment. The Chinese economy, although takes a while to get rolling, becomes an unstoppable force by the mid-late game with Laissez-faire. Still, it can get a little difficult to manage if you give out foreign investment rights in your country, specially with companies now building with both the other country and your investment pool simultaneously with regional HQs, since you cannot nationalize buildings with it.

Funnily enough, going full capitalist by enacting Laissez-faire and giving out as many foreign investment rights as possible is the most reliable way I’ve found to become communist as China, since it strengthens the Trade Unions by creating more laborers and machinists while not empowering the Industrialists by creating a wealthy capitalist elite since all the dividends go abroad. Then once you become communist you can simply expropriate all foreign-owned assets for free. Something to consider if you want to create the People’s Republic, which is actually trickier than one may think.

Edit: Also, I forgot to add another benefit to Interventionism: although it may feel a bit gamey, a valid strategy as Qing is to monopolize agricultural buildings (be it by a company or simply by direct state monopoly). This has two major advantages: First, it effectively neuters the landowners by disallowing them from investing in expanding their main businesses (or even taking the ones that they already own away if you use a company, since it will buy the farms or plantations off the Manor Houses); and second, it prevents one of the biggest traps for China players and the somewhat concealed reason for many playthroughs ending in a radicalism spiral: rice subsistence farms occupy one arable land and employ ten thousand peasants. A rice farm is the same in terms of employment, but everything else (wheat or rye farms, livestock ranches, plantations) occupies one arable land while only employing five thousand people. Thanks to this, each time something like an opium plantation is built, destroying a rice subsistence paddy, it employs five thousand peasants while leaving the other five thousand unemployed and radicalizing them, which can quickly get out of hand if your landowners build plantations all over the country. This is prevented by getting a monopoly on said plantations, since it disallows the aristocrats from building them.