Here’s a few things to consider between the two:
Fit/geometry
SL6 is race oriented and the position will be more aggressive. It has a lower stack and a longer reach than the Roubaix. You may be limited in position if you want to be long and low on the Roubaix depending on where you fall in frame sizing.
Ride
Roubaix has the shock absorption system in the stem and seatpost. If you like a firm, race bike, you might not get along with the Roubaix.
Tire width
Roubaix is designed to fit a slightly wider tire. May be beneficial if you’re interested in riding some light duty gravel roads or if you are wanting a wide tire for more comfort.
My $.02
If it were me in your shoes I would go for the Roubaix. It still pedals really well and feels fast despite the shock absorption system, plus it allows more versatility with the wider tire clearance.
Fit/geometry
SL6 is race oriented and the position will be more aggressive. It has a lower stack and a longer reach than the Roubaix. You may be limited in position if you want to be long and low on the Roubaix depending on where you fall in frame sizing.
Ride
Roubaix has the shock absorption system in the stem and seatpost. If you like a firm, race bike, you might not get along with the Roubaix.
Tire width
Roubaix is designed to fit a slightly wider tire. May be beneficial if you’re interested in riding some light duty gravel roads or if you are wanting a wide tire for more comfort.
My $.02
If it were me in your shoes I would go for the Roubaix. It still pedals really well and feels fast despite the shock absorption system, plus it allows more versatility with the wider tire clearance.