Today, there is a trend to go for two-piece restoration. One pice (base) is placed on the day of surgery on the implant and remains there. the second piece (abutment with crown, or one-piece implant-crow) comes o top.
The recent scientific evidence shows that keeping the biological barrier around the implant and base intact. This limits the chances to see bacteria migrating along with the prosthesis towards the implant - often cause of mucositis/peri-implantitis. See an illustration here: https://www.teeth4all.org/getting-treated-with-dental-implants/complications-with-implants/
With regards to the crown, a one-piece solution where the abutment and crown are 'merged' together present better longevity and less need of repairs (due to porcelain chipping). This option is esthetic enough for prosthesis not too visible when smiling (i.e. posterior region).
For the anterior region, visible when smiling, two-pieces solution offers the flexibility to the dental technician to produce highly esthetic prosthesis. In the anterior region, as the chewing forces are lower, the problem of chipping is less frequent.
See an illustration here: https://www.teeth4all.org/all-about-dental-implants/dental-prosthesis/#Prosthesis-type
Overall the one-piece solution is cheaper than the two-piece one, but the level of aesthetics might be lower while its strength is higher.
The explanation above was for the restorative part. For the implant part, the trend is to use the two-piece part. The rationale is the following:
Having an implant at the bone level and the restoration screwed on top, through the gingiva offers flexibility. Always consider the future development of the patient condition: if there is some gingiva recession, then the restoration can then be swapped for a shorter one and the implant will not show up. New implants/prostheses might be added and, and, being bone level offers more flexibility if the prosthesis must be changed (insertion angle, implant height,...).
With regards to the trans-gingival implant (tissue level implant) thew offer through the assurance that the peri-implant soft tissue will never be disturbed, and thus, the biological seal around the implant limits the risk of bacteria navigating along with the implant towards the bone (cause of mucositis and peri-implantations). this is tough dependent on many other factors as well (implant surface, patient hygiene,...).