Most people use smaller multiple heaters but it depends on what the surrounding temp is too. My sump is in an unfinished basement so I use 2 big heaters and only run the second at night ... for now. In the heart of winter I might have to run both all the time.
Redundancy is a good thing.
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,