Photographylife's lens tests are useful but interpretations of the results tend to be subjective.
I don't own either the 24-70/4 or the 24-120/4 but from images posted that I've seen I've always felt the 24-70/4 has a clarity that the 24-120/4 doesn't have, and sometimes images from the latter seem strange to my eye/brain in how the light and shadow are handled. But this is just my subjective assessment of images posted by strangers, not necessarily reliable! I use the 1st version of Z 24-70/2.8 S and it's my most used lens and most useful lens as well. The reason I didn't get the 24-70/4 in a kit was that I felt it didn't quite autofocus as sure-footedly in indoor conditions as the f/1.8 primes or the 24-70/2.8 (again, 1st version) and I needed the f/2.8 for indoor events. Of the two f/4's I would get the 24-70/4 if you want small (as it collapses) and if you value across the frame sharpness stopped down (such as for landscape photography, architecture etc.) At f/8 in photographylife's tests the 24-70/4 is sharper in the corners at all focal lengths than the 24-120/4. At f/4 it depends on the focal length which is sharper in what area of the frame.
The Z 24-120/4 S seems to have a lot of fans on forums (such as fredmiranda). If getting the 24-70/4 I would buy a lightly used lens or one that has been separated from the kit by a store and sold separately at a lower price than the official new price of the lens. If comparing the two lenses outside of kits, the 24-120/4 seems like it is priced more attractively. A used 1st version of the 24-70/2.8 S might also be considered now that there is a 2nd version luring buyers.
I wouldn't worry too much about the collapsing barrels of the S-line lenses, they are really quite rugged compared with some F-mount lenses of old, which wobbled a lot if you shook them a bit.
The Z 105/2.8 MC is a fantastic lens, an absolute marvel in how the images come out.
