Aiki do or budo?
Art or skill? What is this that we’re talking about? “Do” is the refinement of warlike skills in time of peace. That which is often lost is practicality.
Physical capability
Technical capability
Mental capability
Is there one that is most important? Which can we work on through practice? Is there more?
 Time - We have no choice of our time of birth or our time of death.
Speed, distance, direction - We have little choice of the speed of any given situation. Nor can we alter our own speed beyond our physical limits. The distance and direction of situations are also outside our control beyond that control made possible by our mental capability.
Physical capability - Physical fitness is something that we can always work on. In time of general peace, moderation allows fitness to endure longer. Fanaticism breeds injuries that degrade performance over time.
Technical capability - Within the limits of physical ability, this too may be practiced. Dojo practice is very useful. Martial techniques fall in the general category of ballistic movements which become natural through detailed practice.
Mental capability - This, too, is subject to practice and improvement. Unfortunately, dojo training indulged in for physical purposes, may breed a dojo mindset which is impractical. In general, however, all situations are a product of our mental capability. If our perception improves, so does our awareness of situations, allowing our most advantageous orientation toward them.*
Is there anything else? If there is, it is in the unification of dimensions into a situation. Perhaps it can be experienced. Perhaps a stage can be set. It probably cannot be simulated or practiced. The attempt to do so certainly leads to impracticality in many instances. O Sensei said , “As ai, harmony, is common to ai, love, I call my art aikido.” It is easy to have a superficial understanding of both terms. 
* There is a US military theorem originated by Lt. Col. John Boyd, USAF concerning the “decision cycle”; observe, orient, decide, act - ooda. This is predicated on the existence of a potential situation / state of hostility. Given the highway death toll in America, to say nothing of violent crime, it is a useful perception practice to maximize awareness in daily life and thereby conserve time to observe, orient and decide.