I'm looking for something lighter and comfortable without being flexy.
I read the whole description, I suggested as an option
Read the "comfortable" part.
A Foil ain't a "comfortable" bike. Which given his past poor experience, would rule the Foil out.
is it much heavier / flexier than equivalent Tarmac?
They are two different bikes designed around two different design briefs.
The Tarmac is a race bike, pure and simple. It didn't beat me up when I rode it, but then I race. On the other hand, Roubaix was appreciably more 'vertically compliant' (I hate this term, but really is the simplest way to describe it) than the Tarmac. BUT did not want for any lateral stiffness. Yes that sounds like the 'laterally stiff, vertically compliant' cliche we've been fed by dozens of brands, but the Roubaix really can lay claim to delivering that.
Google is your friend, both frames have been reviewed ad nauseum. I was fortunate enough to be able to ride them back-to-back, so got a great appreciation for the differences in both. I'm certainly not gospel though, so YMMV.