I'm looking at getting an epic but have a couple of options I'm tossing up, so was after some info and possible advice. I ride XC to trail (more xc side though), with a half a dozen or so 50/100km and
4 to 12 hour events a year (for fun, not podiums in the forseeable future).

The 2 options I'm toying with are as follows;

1. Expert Carbon (will change out some of the running gear with higher end stuff or else parts I prefer, which I already have on hand. All up cost isn't really anything much over retail of the bike)
2. S-works frameset and build it up myself (Again, have most of the running gear for the build already, though need a few costlier items than above, so in the end will end up similar in cost to
a Marathon, but if I got a Marathon, I'd be swapping a number of things and spending a fair bit more, hence it's not on the list).

Now the expert will probably be good enough for me, but as I can get either, I was after some info on a couple of things;

1. Apart from the carbon steerer and possition of the brain adjustment, what other differences are there between the fork on the expert and the one that comes with the s-works?
2. How much of an improvement is the carbon rear of the s-works over the alloy on the expert from a performance perspective (ie, pedling efficency or braking or something. I can live with
a small increase in weight, it's the difference in handling I'm wondering about).

I've ridden the s-works full bike (was the final deciding factor in confirming my next ride was an epic), but can't get an expert model to test the differences myself, hence asking here.
So apart from the above specific quesitons, is it worth the extra $$ and hassel to do a full build up, or is the expert close enough not to bother with the frame build?