To get a wider jaw, would a wraparound jaw implant be better, or a lower border mandibular osteotomy with split?

Based on the morphs you have provided, that is only achievable with a custom wraparound jawline implant as it adds width to the jawline....which an osteotomy can not do. No osteotomy can widen the jawline, it can only bring it forward. The effects of soft tissue stretch are greater with an implant than an osteotomy because of its greater volumetric  displacement effect.

 A full set of facial photographs from all angles are required to make a determination about how best to proceed, since the jaw, the mandible, and the implants are  all three-dimensional structures, and the augmentation occurs in all three dimensions. Chin implants are manufactured in large array of  sizes, thicknesses, and shapes. For over 25 years, we used Implantech brand of solid silicone chin implants with an excellent success rate.

The choice of an osteotomy versus implant augmentation can depend upon the 'width' of the front part of the jaw/chin/side of chin area. If the jaw is narrow, then advancing it still leaves less definition/width than having the opportunity to augment with implant material. Given that I do both, the other factor is the simplicity of implant augmentation versus bone surgery. On the other hand, CT scans of the jaw do not allow a proper representation of the overlying soft tissue movement; so it is really preferred to sculpt an implant at the time of surgery rather than rely upon a CT 'estimate'. ie viewing the contours directly at surgery, one can sculpt one side differently than the opposite, since no one is truly symmetrical.