The best is a hybrid of internal immersed coil for low flows and preheat of the a plate heat exchanger, and a plate heat exchanger for high flows.

When you have high flows the DHW plate pump cuts in, otherwise the coil is adequate. This saves on the DHW pump, which can cycle on-off everytime someone just opens the tap for a few seconds, which is quite common. A small bypass around the flow switch with an in-line restrictor sets the flow rate for DHW pump switch on.

The hybrid also gives very high flowrates with the coil preheating the plate.

Another approach is have the coil for kitchen sink and basins and the plate for the high flows of bath and showers only, which can still be pre-heated by the coil.