Hi MG,
The two sets of rules are very different yet very similar.
If you read my review of OTH written shortly after they were published I highlighted the fact that I was looking for a set of rules that would allow me to play games with my collection with friends who prefer dice driven rules over computer moderated ones.
link
The first thing that attracted me to OTH as a possible solution to this situation was the fact that the rules, like C&G, are very much based aground the concept of fatigue caused by combat, movement and all round stress when in close proximity to the enemy. Having played them now several times I find they tick the box for me on my fun/simulation model of analysing rules I like to use, that is I think they provide a high fun, simulation type of game, and they do it in a very slick way that makes them perfect for using them in small or large games without stressing the grey matter, something I have got used to playing C&G.
There are lots of extra rules built into OTH that you can use or leave out, as you choose. We are aiming to play some large games that work very well under C&G to see how OTH copes whilst we use all those great added rules to enhance that simulation aspect. So far we are very impressed and that is why I am very happy to play either set.
As you can see, there is no problem using either set with the collection I have created and I would happily endorse anyone thinking of giving both sets a go.
Cheers
JJ