Arithmetic dictates that Mindbender is obviously stronger. However, the difference gained may not be worth losing the HPS gain from From Darkness, Comes Light. To boot, From Darkness, Comes Light can potentially save more mana than Mindbender gains altogether. I am very confident your best mana management option is to couple the use of shadowfiend and From Darkness, Comes Light over Mindbender alone.
I disagree. There aren't really enough encounters in which you will be using the spells that proc FDCL enough to make it pull ahead of mindbender. If you're spamming single target heals, sure, FDCL is probably OP as all get out, but from what I have seen, even on the fights where that is required, you're doing a fair bit of raid healing as well. It's also hard to quantify mana savings vs a mana return. They just don't function the same way. Although, I may just be biased cause I hate FDCL. I think it's a crap talent, but to each their own.