The uglier painted solid wood skirt on the left will almost certainly way outlast the brand new cheap synthetic stuff on the right. Painting the wood a nicer colour would be better than replacing it with cheap plastic cladding.
Don’t assume that all synthetic skirts are long-lasting and maintenance-free. Cheap brands’ plastics can fade, warp, fissure and crack worse than wood. I’ve seen 25-year-old wood skirts in near-perfect condition, and I’ve seen near-new plastic skirts age terribly.
If you don’t like the idea of having to maintain the natural wood look, then simply paint it with a high-end exterior paint which can last 20 years.
Durability isn’t determined by whether it’s plastic or wood – it has more to do with the quality and workmanship of the cabinet. Nailed-on horizontal pine boards will warp, cup and fall apart in rainy weather, but vertical knotless cedar slats glued and stapled together can last for decades.
Synthetic skirting is more forgiving, in that it can be applied less strategically, and if a panel is made from a single sheet of vinyl, not much can go wrong, except for the eventual wear and tear due to environmental attack. But unlike wood, it can’t usually be refinished – it can only be replaced.