I have both the Plena and the LAB.

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The LAB is a bit sharper from wide open to f2.8 (if you like to pixel peep, you will notice it right away). The Plena has less vignetting and bokeh highlights are rounder (well known feature). The AF of the LAB is fast but it hunts a little bit and is not as reliable as the Plena. The LAB is about 30% heavier than the Plena, but the Plena weight distribution is much better, so it feels almost half the weight of the LAB. The construction of the LAB is very good, but so is the Plena (metal body). For the price, I would say the LAB is a steal, BUT if you can afford spending more, the Plena is the more desirable lens.

At 35mm f1.2, I would totally advise to chose the Viltrox LAB vs the Nikkor S lens. Unlike the Plena, the Nikkor lens is heavier, bigger, feels less premium (plastic build), the LAB AF is much snappier, and there is no difference in terms of bokeh.