So, what's the difference between sumigaeshi and tomoenage? Are these two throws the same thing except with two different "grips" or connections? Your high-powered Trick-Question Sense should immediately start tingling when you read that question, because as I've said before (repeatedly) the old, dead Japanese guys that developed this stuff were not into mindless multiplication of techniques. If two things were not significantly different in principle then they lumped them together and give them one name.
So, now that we know there really must be differences between these two throws, what are those differences?
- name - tomoenage means something like, "circular throw" or "yin-yang-shaped throw," while sumigaeshi means something like "corner counter" or "turning the corner."
- direction - at least as I think of these two throws, tomoenage happens in one axis of rotation(flipping forward), while sumigaeshi happens in two axes of rotation (turning horizontally as you flip frontally). In other words, I usually throw tomoenage in uke's direction of travel, but I throw sumigaeshi close to 90 degrees to uke's direction of travel (similar to the yokoguruma in nagenokata).
- force - tomoenage appears to me to have more muscular insurance associated with it. If you get it close to right, often you can push hard with the strongest muscles in your leg and finish the throw. On the other hand, sumigaeshi often feels like a less forceful, more graceful action that you have to get closer to right because you do not have as much muscular backup.
I have never gotten much mileage from tomoenage. I suppose because of my long legs it is hard to get seated directly under uke and still get my foot in his gut and not in his groin. I do, however have a yokotomoenage that I like a lot, and I get good mileage out of sumigaeshi.