I don't know, I tend to thrive on the subtleties....
I agree that they are similar enough to make transitioning relatively easy, but they will all have certain aspects that seperate them. For example, Seitei Iaido tends towards big movements, and large circles with your shoulders, whereas the Iai portion of Tenshin Katori Shinto Ryu is more direct and tight, so if you perform the TSKSR kata like Seitei Iaido, you've missed the point of Iai in TSKSR. Hyoho Niten Ichi Ryu has no iai section at all, featuring only a formal method of drawing both swords for the Nito Seiho, and they also tend towards tighter actions rather than the bigger actions.
Each will give you the same benefits, though, which when we are looking at swordsmanship, are not really related to self defence, or realistic combative abilities. Instead, the benefits are more focused on aspects of attitude and mindset, but the way they instill these aspects will usually be the determining detail when looking just at different styles. For example, using the systems given, Hyoho Niten Ichi Ryu has it's entire syllabus taught through paired kata, within a realistic distance (or even closer) for the most part. You continually train these kata with your partners, increasing the speed and power, until it's done at a true combat speed and intensity, and thereby forcing you to focus your concentration in the moment. The primary idea is that you never move out of the combative situation until it's over. I have been told more than once that (probably due to my other training) I "like to live dangerously" in terms of how late I leave my evasions and so on.
Within Katori Shinto Ryu, the techniques are much longer and are actually done out of actual conbative distance, in order to allow the kata to be lengthened, and other training devices put in (such as substituting a "blocking" sword for the actual body target you would be cutting or striking, only after training for a while do you get introduced to the actual applications of the techniques you are performing). The techniques are again predominantly paired kata, with the Iai sections and shurikenjutsu being solo. Again, by training the kata with more and more intensity, you are forced to focus your mind and attention to the moment, as if you lose focus or attention for a moment, a bokken to the head is rather an uncomfortable wake-up call. But at least it's better than sharpened steel....
Seitei Iaido is trained solo, and here the mind is again forced to focus on the moment. However, it is not due to another partner who may bean you on the head, but rather in the attempt to get every movement as perfect as possible, constantly refined and polished.
There are then other systems which also have their own peculiarities, such as Tatsumi Ryu, who have a unique method of nukiuchi involving turning the sword upside down first, drawing in a manner more similar to a tachi rather than a katana, showing it's origins. They also have a different method of holding the sword in place, typically most systems, such as Seitei Iaido hold the sword in it's saya (scabbard) with the thumb over the tsuba, but the Tatsumi Ryu holds the tsuba from below with your index finger. Interestingly, Katori Shinto Ryu is actually halfway between the two, with the index finger being used to pull the sword back into the saya and hold it into place, while the thumb rests behind the tsuba, pushing forward to break the seal of the habaki when drawing.
So, really, it's all the same except where it's different!
Small disclaimer here: These opinions are my interpretation based on my emic and etic research and study of various systems, and how I see the value of the teachings. Others may differ, and we may all be correct.
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