318s/320s would make for a better passenger experience on the Inverclyde line, solely because the doors don't automatically open at every stop.
Agree. Would be better for the route, whilst the 314s could be best utilised on the shorter Paisley Canal and Cathcart routes. With only 16 sets, they could be used exclusively on urban network leaving the Gourock route with (marginally) better stock.
Of course, if driver training's the issue then I can understand why that won't happen.
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The status quo is that not all services on-peak are six-car trains, some being operated by three-car trains.
If you get four-car trains, you can replace the three-car on-peak services with four-car ones. If you get three-car trains, you can extend the three-car on peak services to six-car ones.
The time which really matters is the peaks because that's when the largest amount of stock is in use. Sure, some four-car would be nice between peaks, but you can always run them as six-cars with better capacity in the peaks.
Of course, if you introduce 4 car units you can still derive huge benefits. A busy (but not crush loaded) three car train can be extended to four carriages, which then frees up the three car set to double up elsewhere. I can think of quite a few services in Glasgow that were busy with three carriages, but didn't quite warrant six carriages (although things may well have changed).
The downside is that, unless you were to extend platforms, you're creating a micro fleet of four car trains that have to work in isolation. And that will be the problem that Scotrail has. Without the scope to run seven and eight car trains (which are only really possible on Ayrshire without SDO), a four car 321 would be of limited use.