The material you use for heat transfer makes a massive difference. Copper pipe is the king when you consider raw heat transfer and time lag versus dollars . Pex is about 6 times worse per surface area than copper at heat transfer, and also introduces lag. If you have the volume, pex will work (best for slab heating and ground loops). For domestic hot water, and other high drain, not so steady-state processes (quick, high dT/dt pulses), copper pipe is the cool kid.
Rule of thumb for copper is 100 feet of 1 inch will reliably move 100k btu per hour with a dT above 25 degF, steadily, forever. Two parallel runs of 3/4 inch, four runs of half inch, move equivalent amounts of heat. With PEX or poly pipe, the bigger the diameter, the worse the heat transfer. So if you push your luck, half inch might have a factor of 4 or 5 times less effective.
The 1 inch and larger diameter poly pipe has a much, much thicker wall. This thermal resistance and slow flow isn't insignificant. A 1000 foot spool of 1 inch poly pipe will not let you take a 15 minute hot shower with a 150 degF thermal store. Once the stagnant water in the pipe runs out, the water comes out lukewarm. I'm not talking about an eco low flow, locker room showerhead, and a crispy cool shower here.
If you need fast, high density, compact heat transfer, it's hard to beat a brazed plate heat exchanger. If you're thinking about a beer chilling thingy, they work great for sea water. Both are pricey compared to a roll of copper tubing for the raw BTU capacity.
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