Visually
Mhairi is intriguing, but I think I prefer the sound of
Mairi more.
If
Mhairi is usable where you live then go for it. Gaelic names are so charming and this one is stellar.
Siobhan is gorgeous. It would have been my niece's name if my BIL had his way. My sister won and she is
Clare Siobhan, born in the 1980s. We live in the U.S. At the time
Siobhan was unheard of. So I think my sister made a good, if safe, choice. :0)