In reply to ericinbristol:
You're asking two very different questions? People sometimes use the term strength endurance instead of power endurance. Either way, both relate to sustaining a slightly sub-maximal output and 20 in 20 will hit that perfectly as you'll be operating a distinct level down from your max flash level for 20 mins with no rests. In other words, you'll be gassed!
Meanwhile, 15 in 40 is more strength and power based as you'll probably be climbing at around your flash level or just below it.
I'm not sure of exactly how it is defined for climbing, but for me, if I talk about strength, I'm thinking about statically holding holds or a position, whereas power is about movement, particularly explosive movement to reach the next hold. If I can't generate enough momentum to reach the next hold I'm lacking power, if I can reach it but can't hold it then strength is my weakness.