I feel like I ask about spelling variations a lot, it’s partially a matter of curiosity. [name_f]Siena[/name_f] is my latest name crush, and I definitely prefer this spelling (makes me think of the city in [name_f]Italy[/name_f] and positive associations/imagery with that), but I know the double-n spelling is much more common.
Questions: which do you prefer and also do you think [name_f]Siena[/name_f] would be constantly misspelled or mistaken for [name_f]Sienna[/name_f]?
Sienna feels more complete to me. [name_f]Siena[/name_f] would probably be mistaken for [name_f]Sienna[/name_f], but it’s a quick correction.
I find [name_f]Sienna[/name_f] more complete. I can see Siena getting misspelled as [name_f]Sienna[/name_f] but wouldn’t be a big deal - “it’s [name_f]Siena[/name_f] with one n”
I personally prefer [name_f]Siena[/name_f]! This spelling looks more balanced to me and has a sleeker, brighter feel. I think [name_f]Sienna[/name_f] is uncommon enough that people would probably ask how to spell it to be sure, and I think it’s easy to say “[name_f]Siena[/name_f] with one N.”
I personally prefer [name_f]Sienna[/name_f] as it’s my instinctive spelling but [name_f]Siena[/name_f] has a sleek, swish vibe too
I prefer [name_f]Siena[/name_f] for the reasons you mentioned. It is likely that it would often be misspelled with a double-n, but that is easily corrected when it’s important (e.g. a professional document) and ignored when it isn’t (e.g. a Starbucks cup). 
Like you I prefer [name_f]Siena[/name_f], both because of the Italian city and because I generally prefer more streamlined spellings. I think you’d occasionally have people misspelling it but I don’t think it’d be a huge issue.
Siena. I live in the UK, where I have met quite a few Siennas, so [name_f]Siena[/name_f] feels refreshing and fun. However, I don’t love either, I just prefer [name_f]Siena[/name_f].
I like [name_f]Siena[/name_f] because it reminds me of the city in [name_f]Italy[/name_f], however [name_f]Sienna[/name_f] feels a bit more “full”.
Sienna is familiar to me, but there’s something nice and fresh about [name_f]Siena[/name_f]. I quite like the minimalistic style and overall simplicity of [name_f]Siena[/name_f]. I think it would get misspelled by people that had never previously encountered your daughter, but I think it would be a relatively quick adjustment.