Firstly, lets look at their stats. (using Lord numbers)
Charla
6,700 (1,000) HP (7,700 after Imps)
2,500 (340) ATK (2,840 after Imps)
2,500 (340) DEF (2,840 after Imps)
2,300 (620) REC (2,920 after Imps)
Aurelia
7,125 (1,100) HP (8,225 after Imps)
2,165 (440) ATK (2,605 after Imps)
2,290 (440) DEF (2,730 after Imps)
2,540 (440) REC (2,980 after Imps)
Stat wise, Aurelia will have higher HP and REC, the most important stat between the two of them (used to calculate their Burst Heals). Charla has the better attack and defense.... uhhhhh whoopie. However, Aurelia's attack is important since 1) it's used to calculate the damage done by her DoT on her BB as well as 2) her SBB damage. Defense is defense... so Charla wins here.
So now let's go to the buffs they both share; 3 turn BB fill, 3 turn HoT, and Burst Heal.
Charla has 7 BC/turn, her HoT will heal between 2,500 - 3,000 +15% of target's Rec stat each turn, and her Burst Heal will do between 2,700 - 3,000 + 32.5% of her Rec stat.
Aurelia has 4 BC/turn, her HoT will heal between 2,700 - 3,100 +10% of target's Rec stat each turn, and her Burst Heal will do between 4,700 - 4,500 + 22.5% of her Rec stat.
Obviously Charla wins in the BC/turn department, but what about the other two? Let's use JUST their Lord Stats (including Imps) above (so no stat bonus from spheres/ls/ES/etc) and say that the target for their HoT is for themselves (Charla's HoT on Charla, Aurelia's HoT on Aurelia)
Charla's HoT will recover between 2,938 - 3,438 HP/turn, and her Burst Heal will recover between 3,649 - 3,949 HP.
Aurelia's HoT will recover between 2,998 - 3,398 HP/turn, and her Burst Heal will recover between 4,670 - 5,170 HP.
Between the two of them, their HoT are pretty much equal but Aurelia's Burst Heal is far superior.
Now the issue is that their BC/fill buffs are not in the same placement on their BB and SBB. So when it comes to min/maxing numbers, it's a juggling process. You want to flip flop between Charla's BB and SBB each turn to keep up all her buffs to maximize team damage. This means on turns you want to use Charla's BB, you have to use Aurelia's SBB first, so that way everyone gets the 7 BC fill/turn. However, if you want to keep the 7 BC fill/turn buff on, the next turn you're doing Charla's SBB and Aurelia's BB. This leaves you with NO Burst Heal between the two of them this turn. Hopefully that is not a hinderance and causes a wipe. To solve that, you'd want a third source of burst healing just to be safe (Krantz's SBB and Zeruiah's BB are good sources in this case).