Why not both? Large Modular Reactors (LMR). You can produce standardized modules in a factory and ship them via barges to their final destination.
See for example: https://whatisnuclear.com/offshore-nuclear-plants.html
But in a slightly more-plausible vein, Rolls Royce are pushing mid-sized MRs in the 0.5 GW range, with the selling point that they can be built on the location of ex-fossil fuel or 1st gen nuclear power plants.
They're still more expensive per MWh than a modern large nuclear plant, but have the benefit of reusing existing grid connections and other infrastructure. Whether that's actually enough to make them a commercial success is another matter, but there's a chance they could prove a useful stopgap until grid-scale storage costs come down further.
But assembly as well. People learn by doing. The second reactor will be built quicker than the first and third will be quicker than the second.
And you can see it in the Rolls-Royce order book. 3 separate countries have ordered 3 reactors to start with. An order book of 9 reactors strongly incentivises setting up an assembly line.
Those are economies of scale, for what it is worth.