Spirit. Daito ryu is the mental and physical application and view of aiki, the developing yet undeveloped flower.
Aikido is the spiritual, encompassing both the physical and mental, the fully formed.
That may well be true, it's hard to argue that many people were more religious/spiritual than Ueshiba. I have a few comments to consider:
1) Sokaku Takeda and Daito-ryu, although not nearly as spiritual as Ueshiba, are hardly as soulless as they are often made out to be, as evidenced in the scroll above. Did you know that Takeda made extensive studies of esoteric Buddhism?
2) There are only a couple of people that I know of in the world
who are really delving into the spiritual beliefs of the Founder in depth. Despite what many people think they know, what has been translated into English is fragmentary, often mistaken or misleading, and very abstract. Much of the material is specific to Japanese culture, and is difficult even for native Japanese in their own language.
3) There is quite a lot of technical instruction embedded in the lectures left behind by the Founder, but the same caveats noted above make it extremely difficult to extract.
4) That still leaves open the question of differences of a technical nature. Traditionally, the talking points have been that the Founder produced an art of a profoundly different and original nature - not only spiritually, but on the level of technical principles.
On the spiritual side - many of the Japanese and Chinese martial arts had very complex and complete spiritual systems attached, so much so that I'm not sure that the case for a new and unique world view can really be made in it's entirety. There's nothing wrong with not being unique, of course.
On the technical side, I think that the argument for uniqueness in Aikido has traditionally been that it is a new technical paradigm that allows for control of an attacker without causing harm. If this is true (I'm not sure that it is, or that it is even really possible except as a goal), than what are the specific differences in principle? So far, none have been presented except as a matter of choosing to be more charitable - something that other arts, even Daito-ryu, have been known to do. What are the specific and original technical innovations that were introduced by the Founder that are not variations of his earlier training?
In the thread noted above
there is an extremely extensive technical comparison of the two arts. How about an equally detailed review showing the differences in core technical principle?