You have the same amount of pipe in the heatexchanger in both scenarios – there will probably be very little difference in terms of heat transfer / temperature.
For the highest pressure at the shower head, you want the lowest pressure drop possible across the system.
Pressure drop across a section of pipe is proportional to the fluid velocity through that section squared (as stevomcd states above).
Flow through many pipes in parallel = lower velocity through each pipe = lower pressure drop through the heat exchanger (compared to a single long higher velocity pipe coiled around). This gives you the lowest pressure drop, and greatest potential flow through the system.
However, more flow through (basically) the same heat exchanger with the same heat source = lower temperature.
Get more pipe in that heat exchanger!