I had an Epic Comp carbon which I sold last fall. I converted it to Evo. Fitted a dropper from the get go, as I'm used to it from my trail bikes, and the weight penalty is not really much. I then fitted a 120mm Revelation by chance, as the Reba it came with imploded.
To be fair, for normal rides (both trail and XC), the standard Epic is seriously capable when fitted with a dropper. But I always felt I had to go more carefully with the 100mm fork. The more solid 120 will let you attack harder in the rough, and the slacker head angle also helps a bit on the descents. I didn't feel like the extra travel made the bike any slower or less snappy.
For all-day riding, I'd go with the longer fork. Also if you do long trail races, like BCBR. I did a few trail ultras with it, and the 120 front made it more comfortable and easier to ride.
But, if you're going all-out for sub 2-hr trail racing, 100mm is enough.
Kinda miss it...sold it just when the Evo arrived over here.
BTW, the Expert Evo comes with a lovely set of wheels, making it even more burly. The standarsd wheels on my coimp carbon were heavy and soft, so I ran Roval carbon wheels.