[name_f]Tilly[/name_f] for a full first name. [name_f]Tillie[/name_f] for a nickname.
I like [name_f]Tillie[/name_f]… Like [name_f]Millie[/name_f]. There’s a clothing store here called [name_f]Tilly[/name_f]'s… so maybe more people would automatically spell it as [name_f]Tilly[/name_f] because of that? I’m not sure.
[name_f]Tilly[/name_f] would be better if you are going to use as a stand alone name, otherwise I actually prefer [name_f]Tillie[/name_f] as a nickname and will be using that for my future daughter [name_f]Matilda[/name_f].
I like the way [name_f]Tilly[/name_f] looks written out in cursive, so that gets my vote.
The -ie ending is helpful to distinguish girls nn’s from boy nn’s – [name_u]Billy[/name_u] vs. [name_u]Billie[/name_u]. But since [name_f]Tilly[/name_f] doesn’t really have that problem I think the y is simpler and lovelier.
I prefer the [name_f]Tilly[/name_f] spelling.
Mostly because I think the Y gives more grip on names.