The high pressure Challenger is a higher head pump than the high flow version so there is a difference in head curves. The higher flow version will have a lower flatter head curve while the high pressure version will be higher on the right and steeper on the right. The higher flow versions are usually slightly more efficient but it depends on your plumbing. For larger pipes, the high flow version may give you more flow rate/watt while for smaller pipes, the high pressure version may give you more flow rate/watt. But since they are both low HP pumps, it may not matter all that much either. High flow pumps are usually used for waterfalls where high volume and low head are required.
Have you considered going with a two speed? That would save quite a bit in energy costs. You could get a two speed Whisperflo (WFDS-24) for only $100 more than the Challenger.
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