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If it has come down to these 2, my pick is [name]Fianna[/name]. While I do love [name]Seraphina[/name], it is associated with [name]Jennifer[/name] [name]Garner[/name] and [name]Ben[/name] Affleck. Also, I think I read that your name is [name]Sarah[/name]? In the middle spot it would be cute, but it is too similar to your own name for the first.
[name]Fianna[/name] matches well with [name]Peregrine[/name], I think.
All of your children have beautiful names btw!
I prefer [name]Seraphina[/name] and don’t care about celebrity associations too much, but I think [name]Fianna[/name] feels more like the long form of [name]Fia[/name]. However, like you said, you have [name]Eve[/name] with a longer middle name, so why not [name]Fia[/name] with a longer middle name? I’d push for that if I were you.
I prefer [name]Seraphina[/name] — all your kids have unusual but established names; [name]Fianna[/name] is trendy and made-up in comparison.
For the nickname [name]Fia[/name], I’d suggest [name]Fiamma[/name] or [name]Fiammetta[/name] —*meaning “flame” in Italian and used as names there.
Yeah, I just did some Googling about [name]Fianna[/name]. I thought it was more widely used as a name but it seems to be more of a folklore word meaning ‘soldiers’ and a modern day political party name.
I like both! I think I prefer [name]Seraphina[/name] though, but next to [name]Eve[/name], I have to go with [name]Fianna[/name] simply because it’s shorter. Two lovely names [name]IMO[/name]! 
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Hm. I was going to go with [name]Fianna[/name], but if it’s true that it wasn’t really used much as a name that would influence my opinion in favor of [name]Seraphina[/name]. (Now why on earth is my computer’s auto-correct accepting [name]Fianna[/name] but underlining [name]Seraphina[/name] as misspelled? Get with the times, spelling software!)
Ahem. Back to the issue. [name]Seraphina[/name] is a bit frilly for my taste, but that really is inconsistent because I like a few frillissima choices such as [name]Christiana[/name] and [name]Georgiana[/name]; it’s a bit celebrity-associated, but you said that doesn’t bother you (and the only reason I knew [name]Jennifer[/name] [name]Garner[/name] and [name]Ben[/name] Affleck used it was because of Nameberry); and it has the sound of your name in it, but your husband likes that, so it could actually be a plus. And it’s [name]Fia[/name] you want, so if [name]Seraphina[/name] will make you both happy, go for it!
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[name]Saoirse[/name] is actually alright as it’s genuinely used as a name, Irish actress [name]Saoirse[/name] [name]Ronan[/name] will be bringing it more into use too I imagine. Yes it means ‘freedom’ but I don’t think it’s as political as it seems. 
I understand about the spelling issue though.
I love your lace-descriptions of the contrast between [name]Christiana[/name]/[name]Georgiana[/name] and [name]Seraphina[/name]! They’re perfect. You’re right, [name]Seraphina[/name] is so…light, so fluffy.
Actually I can see you with a little [name]Fiona[/name] more readily than a little [name]Seraphina[/name]. Was the Shrek reference the only thing holding you back from [name]Fiona[/name]? I agree it’s unfortunate, but I don’t know if it should be enough to make you toss the name. If she goes by [name]Fia[/name] day-to-day anyway, that reduces the number of people who’ll think of the ogress.
[name]How[/name] about [name]Sofia[/name]?
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Grrr…silly Dreamworks. I suppose your husband’s right. Are you looking for some frilly middle name suggestions for [name]Fia[/name] now?