What they share
Both are short, punchy one-syllable names. Both travel well. Both feel equally at home on a toddler and a thirty-year-old. Neither demands a nickname, which gives them a certain confidence.
Pick Finn if…
Pick Finn if you want the Celtic, nature-leaning option. Finn comes from the Irish Fionn, meaning 'fair-haired' or 'bright', and carries the legend of the mythic hero Fionn mac Cumhaill.
Pick Jack if…
Pick Jack if you want the classic English everyman. Jack is the diminutive of John ('God is gracious') and has been one of the most consistently popular English boy names for centuries.
Finn is a little more modern, a little more distinctive. Jack is a little more traditional, a little more universal. Both are bulletproof choices.