You've already made your choice, but I figured I'd chime in since I have both a roubaix and a LHT. First, you made the right choice. Never buy a new bike right before a 500 mile trip. You would regret it unless you got it dialed in perfectly. In my experience, the LHT is perfect for me for riding fat studded tires in winter, and for hauling a weeks worth of clothing, a laptop and lunch. The ride itself is very smooth and I enjoy it tremendously in all sorts of weather. I do not ride it for club rides or on supported rides unless its going to rain.

The roubaix is a much more fun bike to ride. It handles well, is comfortable over long distances and is significantly faster than the LHT. The only reason I'd suggest you buy a new LHT is if you want to do touring or commuting significant distances with lots of stuff. For your charity ride consider a really big seat-bag along with a handlebar bag. If the ride really is fully supported, you don't even need that.