Fighters and Soldiers rarely reach 17 AS, if I recall. It's only like 1-2 of the higher-levelled ones. The only ones that consistently reach that, and more, are Myrmidons (who deal neglegible damage). A trained Aran (aka the only reason you're bringing him to 1-E in the first place) is massively better than Fiona, even if he's getting doubled.
I'm going to compare them, assuming some levels and resources on them.
I estimate you can get Aran to lvl. 14 by the start of 1-E, even assuming somewhat a of brisk case (not necessarily LTC, which adds more points):
lvl. 14 Aran
HP: 32 (Angelic Robe)
Str: 15
Skill 17
Spd 12
Luck 8
Def 18 (Dracoshield)
Res 3
There's two Angelic Robes, and he's a good candidate for things like Secret Book and Dracoshield, for how quickly he caps Skill & Def. They're also fairly uncontested and just make his strengths, stronger. This is also without any BEXP. I counted around 2 Soldiers with 17 AS in 1-E, and they do between 24-26 Atk, and that's only 8x2 in the worst case scenario. Fighters don't double him, and there's only one Archer that's somewhat of a threat (one 17 AS/28 Atk Archer.
Since his affinity is also Thunder, he could easily have a +2 Def by this point.
Fiona,
lvl. 14. Yes, I'm assuming we're actually slowing down a lot... enough to give her 5 levels in only one chapter, since she literally doesn't exist until 1-E:
HP: 32 (Angelic Robe)
Str 12 (Energy Drop)
Skill 12 (Secret Book)
Spd 13
Luck 10
Def 13 (Dracoshield)
Res 9
She'd have virtually the same Spd as Aran, at the same level. That 1 extra point doesn't save her from the the outliers, and she's taking a lot more damage for it, even considering she took a Dracoshield. She could even take an extra point of Spd/Def through BEXP, only to take slightly less damage, but against 10% of the enemies. And it's not likely she'd have any supports to rely on by 1-E, so she'll still struggle with things like Atk, Hit and just about everything else. Even her supposed advantage of having a horse doesn't exist, because indoors. (thank you FE10).
I didn't mean to turn this into a debate, but it's just worth noting that Aran is far, far better than Fiona... or it's just that she's that bad, even when you turn things in her favour. Even Meg could be significaly better, considering all the chapters she has, and she's at least somewhat of a strong tank if you manage to get her there.