The J Boats J/121 2017 vs J Boats J/124 2005 comparison sits squarely in the category of decisions where specs alone won't tell the whole story: intended use, storage, and long-term ownership costs all factor in.
On paper these two are close siblings in the size department - J Boats J/121 2017 at 40,0 ft versus J Boats J/124 2005 at 40,0 ft. At 11 464 lbs and 11 500 lbs respectively, both sit in a similar weight class - either should pair comfortably with most mid-size SUVs and half-ton trucks, though always confirm your specific tow rating with the motor added.
Both boats are rated for 12 passengers - a good fit for a family of four or five plus guests. Comfort at capacity is another matter; the longer hull typically means more seat options and better weight distribution.
Both boats sit in a similar displacement bracket - 11 464 lbs for the J Boats J/121 2017 and 11 500 lbs for the J Boats J/124 2005. Comparable displacement means broadly similar seakeeping behaviour and load capacity, though hull form and ballast ratio will still produce noticeably different sailing characteristics.
Both boats draw a similar depth - 7,1 ft and 6,9 ft respectively. Marina access and anchorage options should be broadly equivalent between the two.
The J Boats J/121 2017 uses Sloop rigging.
Bottom line: The J Boats J/121 2017 and J Boats J/124 2005 are closely matched on paper. A sea trial on both in representative conditions is the only reliable way to find which one suits your sailing style, home port, and intended cruising ground.