> Are the OEM's just the inserts while the aftermarkets are body and insert?

That was my initial suspicion. My understanding is that both are
complete struts (not the full assembly ie without the spring, new
mount but still the complete strut). My knowledge here is fairly
limited so I'll point you in the direction of the diagram of the OEMs
on Honda's website. Here is link to the diagram

http://estore.honda.com/epc/images/p.../S5A3B2800.png

The part which I am referring to is part number 5 (and 6) in the
diagram. My understanding is that the insert would be a inner portion
of part number 5 and is also an option to be purchased separately.
However, my preference is to go for the replacement of the complete
strut.

Feel free to correct me if my understanding is incorrect.
Cheers.