Various thoughts...
1) Diffraction at these magnifications at f/16 will significantly effect sharpness. If you repeated the test at f/11 on the MF system you'd find even greater difference. Whether this is relevant to your work (whether you could ever capture at f/11 on MF based on DOF requirements, style of shooting, willingness to focus stack etc) is an important but separate question.
2) If you're looking for large improvements in single-shot capture you'd want a system which allows tilt and swing (e.g. a view camera with a digital back rather than a fixed lens SLR).
3) In my experience the Hassy 120mm macro shows diffraction earlier in the aperture range than the Phase 120mm macro. Possibly because of the non-optional use of a leaf shutter which adds additional surfaces for the light to diffract around (I'm not sure of the underlying technical cause but it shows repeatedly in practical testing). A Mamiya based solution would provide a less expensive body, and less expensive macro lens, with - in my biased but informed opinion - better characteristics for jewelry.
4) Starting with "I see a good deal on XYZ, will it work well for me?" is, in my experience, a recipe for disappointment and frustration. If you're interested in medium format you should start by asking "What would work well for me?" and THEN look for a good deal on that equipment (including the value of warranty, condition, support, training, accessories and accessory condition, cost of additional items you're likely to want in the future like backup bodies, other lenses, cost of repairs or rentals/replacements/loaners if you have any issues etc). The world is large, and unless you need something tomorrow then you'll find a good deal on whatever equipment you find by research/testing/opinions; jumping on a particular deal you see that you think is especially good - unless it is absolutely exceptional - is giving yourself an artificial and dangerous deadline.