Cup and cone bearings: better a little tight or a little loose?

As loose as possible without play. Bearings are supposed to have some amount of preload, which is a fancy name for being a bit tight as opposed to being loose. There should never be play once adjusted and on the bike.

For QR hubs, a good target amount of play off the bike is such that the play can be felt on the bike with the QR 50% closed but not 100%. Some people use tools either commercially available (the Stein axle vise) or improvised to adjust the hub while under QR compression. That is technically better but slower.

Without preload, the distribution of load is uneven across the bearing race surfaces, and they become worn prematurely.

Some people talk themselves into the idea that a bias towards loose is okay in some circumstances, but it is not. One of these scenarios is when prior damage (pitting, brinelling) or a bent axle is creating a situation where you have a rough spot unless you make the whole thing loose. The correct answer is still as loose as possible without play. Neither is perfect in this situation, but if you give it play, the overall condition will get worse faster.

you retry you'll get a bit loose, sometimes a little tight, never the in between perfection.

For some bikes perfection is not possible.
Like when working on a brand new or used box store bicycle or when working on a beat up bike and you don't have the parts you need right now....

If my only choice is between too tight and too loose it's best to be a little loose.

Too tight chews up bearings, bearing surfaces and often delivers a poor riding experience.

Too loose is can also be hard on bearings and bearing surfaces but in a different way. And, at least it will be a better ride for a little while.

It's best to exhaust all possibilities before giving up on perfection.