Try them at various stages. They are edible at any stage, but turn "seedy" and bitter when they are old.

My bet is that the white or green-striped will be best.

Yellow isn't a good thing. You can harvest them when they are golf ball size and white. The younger they are, the smaller the seeds.

I just pick them when they're still white, even though they're still very hard and firmly attached to the plant? Right, I'll give that a go, and try the yellow one as well to see how it cpmpares. Thanks for the advice!

Eggplants will lose their sheen at the point where they begin to develop seeds. They can be any size at this point. I find picking at this point is when the fruit is as large as it is going to get and still have a good flavor. Picking at an earlier stage is surely an option and does provide a better product.