There were huge and significant differences between the principate and dominate.
1. The empeor's public image during the principate, even though it was far from the truth, was one of "first citizen" during the principate, where he was a person and fairly accessible (see how a lot of a emperors went around resolving judicial disputes and talking to subjects). The fact that the senate was long-since irrelevant power-wise doesn't make this framing less important as a basis ofr legitimacy.
Whereas under the dominate, a lot of ceremonial and formal barriers to interacting with the Emperor were thrown up, to try and distance the emperor from people, emphasize the pseudo-divine nature of the emperor, and generally re-arrange the legitimacy with a more proto-diving-right sort of notion, rather than ostensible first citizenship.
2. Seconly, the principate ruled in a substantially different manner than the dominate. The principate had an astonishingly small bureacracy for how large the empire was, basically only a bare-handful of people running the emperor's household and not much else, with local elites handling a lot of matters in provincial cities.
The dominate put a much, much more centralized and larger bureacracy, with many, many civil servants, moreso in the east than the west (this is partially why the east kept on chugging while the west collapsed, among 180,000 other reasons) but overall the empire under the dominate relied much less on local elites for administration and was far more centralized. This meant the economy of the late empire was way more integerated, the armies were larger, and the government had more control over affairs throughout the empire, compared with the comparatively (comparatively, mind you) light touch and absolutely tiny ruling bureacrac of the principate.