I do my initial focusing wearing a relatively strong pair of reading glasses. But I'm often amazed just how much I'm off once I introduce the Peak magnifier.
The advice to just stop the lens down more, for sake of more depth of field, doesn't always work ideally. Sometimes I want a shallow depth of field, especially when printing stacked negatives, for example, the primary negative plus registered film masks. A shallow depth of field pertaining to the neg carrier means that secondary dust, film grain, Newton rings, or minor flaws in the carrier glass and less likely to be reproduced in the print.
I'll let Vaughn answer S. in his own manner, since he often contact prints. I can just state that there truly are subtle differences which normal vision can pick up when an image is optimized and worthy of close inspection. There's a feel to it. It's much like how, even in say, my 16X20 prints, although the ones enlarged respectively from 6X9, 4X5, and 8X10 negatives might all have the same amount of sharpness, detail, and even contrast, they still feel different to me.
Logistical concerns might dictate which equipment is best in the field, but even there I find myself strategizing the formats somewhat differently. There are certain intangibles difficult to quantify, which still register with a sensitive viewer. And those aspects we might classify as microtonality and subtle edge effect comes across best when the enlarger focus is spot on.
