Interesting and good question!
I too work both ILC and Dan's Ueshiba Aiki
and got introduced to this whole area via purchasing Aunkai DVDs. I'm to much of a newb to really comment on similarities and differences but here goes - not so much as specific methodology, but just things I've recognized.
disclaimer alert - I'm newb so what I write below is probably wrong!
First off in terms of feeling
I see a lot of overlap between Sifu Chin and Dan, but now that I'm learning I'm not to surprised by this.
Terminology - you have to be careful cause I think Sam and Dan say the same thing but in different ways and when attending seminars I want to avoid confusing two methodologies. For example Dan talks of opening & closing
the body and the exercise is a stretching/removing the slack of the myofascial system to feel the connection to dantien. In ILC I find this to be the Expand & Condense
exercise. Whereas in ILC opening & closing
refers to the relationship of the wrist/elbow to the shoulder line.
Next for me is my overall martial goal. I've done Japanese weapons work in the past, but its not my thing. I've heard Dan speak to applying the Ueshiba Aiki work to Koryu weapons work. Dan and Sam are both experienced empty hand fighters, but my interest and background in kempo/grappling/boxing make me embrace the ILC system more and you'll find ILC guys going out and competing so working with them adds to the learning ladder for me. From my brief conversation with Dan, he's an MMA guy before there was MMA but most of the local guys practicing his exercises are in Aikido dojos and so the direction is focused there.
Regarding Aunkai, be careful with the exercises. There are details not explained in the DVDs. Fortunately there are a couple guys in Seattle who hosted Ark in the past and explained some of them to me. If Tenshijin is killing your back I'd be concerned you're doing something wrong - i.e. not stopping at a body limit and letting you're structure stretch and slowly adapt. but I could be mistaken.
I haven't attended a Mike Sigman seminar, but don't intend to - my plate is full of good stuff as it is!
my 2 cents worth!