Both are lovely names, but I definitely would choose [name_u]Florence[/name_u]. It has the same vintage charm as [name_f]Mabel[/name_f]. Sisters named [name_f]Mabel[/name_f] and [name_u]Florence[/name_u] is just perfect!
I may be slightly biased though as one of my daughter’s middle names if [name_u]Florence[/name_u] and we are currently expecting our third who will most likely be [name_f]Mabel[/name_f] if a girl.
Good luck!
ooh [name_u]Florence[/name_u]! [name_f]Florrie[/name_f] is a stunning nickname choice 
[name_f]Eliza[/name_f] just doesn’t shine as much for me, and feels a bit too common next to big sister’s name! [name_u]Florence[/name_u] [name_f]Eliza[/name_f] also flows a little bit better imo, and shares the -a ending with Mabel’s middle, which is a sweet connection!
[name_f]Eliza[/name_f] [name_u]Florence[/name_u] as a little sister to [name_f]Mabel[/name_f] [name_f]Sophia[/name_f] gets my vote !!
Congratulations! I really like [name_u]Florence[/name_u] [name_f]Eliza[/name_f]. I think that’s a beautiful flow and fits really nicely with [name_f]Mabel[/name_f] [name_f]Sophia[/name_f]
I much prefer [name_f]Eliza[/name_f], but [name_u]Florence[/name_u] is also lovely and goes better with [name_f]Mabel[/name_f] imo. You can’t go wrong though, both great choices. I adore [name_f]Eliza[/name_f] but would be worried about [name_f]Ellie[/name_f] nn.
Congrats! 
I’m in the minority and prefer [name_f]Eliza[/name_f] with [name_f]Mabel[/name_f]. [name_u]Florence[/name_u] is a lovely name she’s charming and I like [name_f]Eliza[/name_f] & [name_u]Florence[/name_u] equally but I prefer the fun vibrancy of [name_f]Eliza[/name_f] with [name_f]Mabel[/name_f]. [name_f]Eliza[/name_f] is a lovely vibrant name that feels fun. To me she has a similar vibe to [name_f]Mabel[/name_f]. I also like that both names are 5 letters I think that makes a really nice sibling set.
I would love [name_f]Eliza[/name_f] [name_u]Florence[/name_u] & [name_f]Mabel[/name_f] [name_f]Sophia[/name_f] 
Congratulations! [name_f]Eliza[/name_f] [name_u]Florence[/name_u] is my preference, but both are lovely.
I much prefer Eliza! It’s so vibrant 
I think both names are lovely but [name_u]Florence[/name_u] goes best with [name_f]Mabel[/name_f] 
[name_f]Florrie[/name_f] is a cute nickname as well, [name_u]Florence[/name_u] [name_f]Eliza[/name_f] is lovely and I like that the syllables compliment Mabel’s name as well. They go together perfectly without being too matchy, it’s lovely.
I think to me it depends where you live. I feel like [name_u]Florence[/name_u] here in [name_f]England[/name_f] is very expected now. While [name_f]Eliza[/name_f] is popular, I hear it less. From looking at the stats though, I assume it’s very much the other way around in the US.