The 2017+ Roubaix rides on very similar geometry as the Tarmac, with the Future Shock stuck on top of a taller headset. It's a surprisingly agile, racy bike, which always feels like it's riding on roads made of feathers and foam. It's amazing. If I had to have only one bike for riding and racing, I would much rather have the Roubaix.
There are plenty of other great bikes out there, but ride a Roubaix on some cobbles, or over a few railroad crossings, and I think you'll see what I'm saying.
Also, it's not a "maybe" — the Roubaix has room for 28c tires easily.