First off, welcome to the board. I've learned a LOT in the few months I've been participating.
You ask an interesting question, and one I'll follow closely since I'm hoping to upgrade next year.
My current steed is the Trek 620 that I bought in 1983 so I'm afraid I can't provide any first-hand insights!
A recent thread elsewhere on BF suggested that the Roubaix geometry was a bit more upright than many higher-end bikes, which was good for folks that don't care for a head-down riding position. So that may be a factor in your choice.
Is the Madone available in the "H3" geometry (somewhat more upright than the usual "H2" geometry).
I would suspect that either frame could handle your weight without difficulty. But I'm curious what the more knowledgeable folks think about the wheels.
It may be that going to a higher spoke-count wheel would be the best way to ensure robustness for us in the 200+ club. I've been as high as 240 lb but my 36-spoke wheels stay true with little or no attention (Mavic Open-4 CD's bought in ~1994).
Or to put it differently, the wheels are likely to be the place where our weight will give us equipment problems.
Your choice may depend on how you feel about having your wheels trued on a more frequent basis; I find it a PITA to take things to the LSB for attention, so a robust-but-heavy approach works for me. If you can do your own truing or don't mind having the LSB do the work, then your calculus may be different than mine.
-Tom in SoCal