What they share
Both are deeply classic English names with centuries of continuous use across English-speaking countries. Both have royal and literary namesakes. Both feel confidently normal without ever feeling dull, which is a harder trick than it sounds.
Pick Jack if…
Pick Jack if you want the shorter, warmer, less formal option. Jack began as a medieval nickname for John and has long since stood on its own. It lands with an easy friendliness and does not need dressing up.
Pick James if…
Pick James if you want the longer, slightly more formal classic. James means 'supplanter' and has been worn by apostles, kings, and presidents. Jamie and Jim offer softer short forms if the full name feels too formal for everyday use.
Jack is the friendlier, more immediate name; James the steadier, more ceremonial one. Both work in every setting and every generation. If you want flexibility later in life, James gives you more to work with; if you want warmth now, Jack is hard to beat.