The Difference Between Judo and Aikido Ukemi
Here is a nice demonstration of the classic Aikido form of Ukemi. Let me begin my comments with an admission that I am clearly biased, with over 30 years of Judo practice and only about 2 years of cumulative Aikido experience. That said, I have found the precise application of the Judo style to be more protective than that of Aikido. I have observed two fundamental differences, each of which is highlighted by comparing the various demo videos posted on this site.
First of all, the Aikido version has Uke tuck the off-direction leg under the other closer to the buttocks. This has the effect of somewhat facilitating a roll to a standing position, but it fails to protect either the ankle bone, or one leg from impacting the other upon significant impacts. This leg tuck also has the effect of decreasing the angular moment of inertia, and can do nothing to slow the rate of rotation and thus better control the landing position.
The other fundamental difference is that the Aikido style seems to rely on being relatively in control of the fall from the beginning with more use of hands for gentling the falls than does the Judo variation. Ultimately, I have found that the Judo techniques can protect a body from a wider range of falling situations that start out farther from control.
Those very minor details aside, it is clear that the fundamental principles are very similar, and it is very possible that there are larger differences from instructor to instructor than between the two arts.