I think [name_f]Isadora[/name_f] looks more balanced 
[name_f]Isidora[/name_f] looks wrong to me for some reason. So definitely [name_f]Isadora[/name_f]!
[name_f]Isidora[/name_f] reminds me of the male counterpart [name_m]Isidore[/name_m]. So I think that works but visually I prefer [name_f]Isadora[/name_f]
I prefer [name_f]Isadora[/name_f], I think it looks more feminine (and prefer [name_m]Isidore[/name_m] v [name_m]Isadore[/name_m] for a boy!)
I knew of an [name_f]Isidora[/name_f] a few years ago before I knew this name existed at all! so that spelling looks like the βrightβ one to me! I also think the i makes it look prettier written down !!
I still think [name_f]Isidora[/name_f] is best. The name relates to [name_f]Isis[/name_f], so the spelling is a tad less made-up to me. Also βSidβ is a possibility where with [name_f]Isadora[/name_f], you only have sad. All I see is sad.