I rode the 203 mile Seattle-to-Portland on my Cannondale Synapse carbon fiber (it's very similar to the Roubaix.) I also have a touring bike. I had attempted the STP the prior year on the touring bike. For a one-day ride where you don't have to carry much, I'd much rather ride the lighter bike. I did switch the saddle. I put the brooks b-17 on the synapse. I carried everything I needed in my jersey pockets. If you think you need to carry more, consider a handlebar bag and/or a saddlebag (I have a midsized carradice pendle bag that attaches perfectly to a brooks b-17 saddle.)

I think rando specific bikes are really cool. But, until I am fit enough to do 400+ km brevets on a regular basis, I'm sticking with the cannondale synapse for long one-day rides. As far as lights go, I'm riding with a magicshine ($80) and dinotte (on my helmet - $150) combination, and a couple of planet bike superflashes for the rear. They all run on batteries with a max runtime of about 3 hours on high. Three hours of riding in the dark is enough for me.