IIRC, very early runs of the SP101 had a shorter frame and cylinder than the current SP101, and were thus a bit lighter and shorter than current SP101s.
When the first SP101 .357s were brought out on that same frame, they were marked "For 125gr bullets only", as the length of the cylinder precluded use of heavier .357 bullet loads.
If you plan to shoot .38s only, I'd suspect that it's theoretically a better choice than a current SP101 chambered for .38Spec only.