It might be obvious, but if you want to use a GHB hero at 5*, level up the 3* version for more SP.

Most people find that leveling from 4* Lvl 1 to 5* Lvl 40 is still not enough SP for all the different SI you want to use on a main-party unit. So if you want to use a GHB hero (like Camus/Xander), it's wise to start from the 3* version and go to lvl 40 at both 3 and 4* to farm the level-up SP.