I like them both, but prefer Domhnall or its more modern spelling Dónal.
It does depend on where you live of course, but of the two, Domhnall is easier to pronounce - at least, once you've been told how (if outside Ireland). The 'ch' in Donnacha is tricky to get right even within Ireland, and you'll often hear an approximation of the true sound.
With regard to the spelling, I don't think a fada is needed in Domhnall as 'omh' can give the same pronunciation as ó. The name seems to have been spelt without a fada originally, and the surname is spelt Ó Domhnaill too. I have seen an accent on the Scottish form of the name, Dòmhnall, but usually it's just Domhnall in Ireland (though the spelling Dónal is mostly used now).
Donnacha is often now spelt as you've given it, but the first 'a' was added to reflect the pronunciation, and the spelling Donncha is probably truer to the original, if it matters. (They're pronounced the same.) That said, the name has been rising in the charts over the last 20 years or so, and Donnacha has been by far the more popular spelling over this time.
I am older, so I know more people called Donál than Donnacha, but trends have changed.