Hmm, how do I explain this.
Body Kouros (and all this talk has made me get my bottle and give myself a good coating of it) gets its sweetness from a note that is almost buttery and creamy, for want of better words, thick and heavy. Plus since its Benzoin there is an almond/marzipan thing in there at the start if you concentrate. This sweetness is tempered by the spicy and smokey aspects and turn the whole scent into one of the great masculines, but its still 'thick'. Its a brute of a fragrance (not Brut, but a brute), a powerhouse oriental, and thats why I like it.
Rochas Man seems to get its sweetness from sugar, and then the Mocha gourmand aspect is added, which doesn't really make it sweeter, just more foody. The sugar sweetness doesn't come across with that thick and heavy vibe. Then the Lavender, which is quite herbal, seems to lighten the smell further.
You can easily justify owning both, about the only thing they have in common is that they are sweet, personally cannot see why this comparison has come up so often (see the Similar Threads below). I dont have RM, but I would have both, except that I'm looking at some other options for a chocolate based fragrance.