My opinions are probably heretical, but I'm going to post them anyways.
Firstly, as for the Space Marines power armor vs Sisters power armor:
IMO Power Armor should give 3+ save and +1
Str, regardless of who uses it. The black carapace makes you faster and have a better reaction time, this translates to a bonus to initiative, and maybe an extra melee attack. Relative to Sisters, Space Marines should have +1 I, +1 A and +1 T. The whole
Minus Four Strength this is moronic. The difference between an unarmored unaugmented human, and and unarmored unaugmented Space Marine should be less than the difference between a normal unaugmented human with armor vs without. So that's obviously the first thing I'd update.
As for the actual fluff, I look at miracles from the perspective that ALL SUPERNATURAL ABILITIES COME FROM THE WARP. Which means that faith powers are weird psudo psyker powers. Sisters seem almost like nascent Daemons to the Emperor's Nascent Warp God, and should be stated as such. 6++ compared to daemon's 5++, and more subtle abilities compared to obvious psyker craziness of Chaos. But the Emperor opposes Chaos, and the Sisters Abhor the Witch. Which means that they should be among the best anti-psyker armies out there, but instead, have merely a +1 to deny the witch, assuming that said power is targeting them,
and assuming that they've got any dice to spare.
The wargear for anti-psyker units already exists in the Inquisition Codex, the Psyocculum and the Null Rod both are things that should be either available to special characters, or rolled into a special character. The core battle sisters are they are now are fine. That is, the
BSS, Dominions, Seraphim and Celestians (Though melee Celestians would be cool.) Giving the Celestians a drop pod so that they can get rending heavy flamers into relevant places quickly, is probably worth doing though.
Celestine should have Eternal Warrior, but since I'm the one in charge, I'm going to go one further, and have her upgraded to something like they did with the Ork that became a Lord of War, and make her a Flying Monstrous Creature. Or maybe have a non-named saint that can be customized like a mini-daemon prince, all angelic and sometimes with wings. This is our laser eyes miracle unit, which lets everything else be more subtle.
The Canoness should be able to take a jump pack, and the Command Squad needs someone better than me to make it worthwhile.
Repentia are cool in theory, but need something that lets them not melt when something looks at them, someone mentioned 5++, which I think works, and their act could give them a 3++ for a turn or something.
I'd stick the dedicated anti-psyker into Elites. Something Akin to the Sisters of Silence, and maybe an Independent
HQ that doesn't use up a
FOC, that can hop into other units similar to what a Hereticus Inquisitor with Null Rod and Psyocculum would be able to do.
Instead of creating a whole new unit for anti-air, an alternate miracle for the Seraphim could work instead (Sister Superior Upgrade/Sidegrade?), let them act like flyers for a turn, this gives them good reason to take the 2x double Melta again.
As for the Penitent Engines, I don't like them, so I haven't thought about how to make them worthwhile, nor will I.
