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.

No comment on the Tarmac but I have had a Roubaix. It was a great bike. Very versatile and even capable of handling some light gravel. Never felt too heavy despite its higher weight, position isn’t too upright (it’s modeled on an older tarmac model) and the comfort is very good. A definite all rounder and if you planned on racing you can lay around with the position and put some racier wheels on it.

My Roubaix is incredible and with some news light pieces and wheels it’s under 7kg

Either will work for you, the Tarmac has slightly sharper geometry and better suited for racing, the Roubaix slightly slacker geometry which often makes it a slightly easier bike to ride - or slightly more relaxed depending on your viewpoint. I've owned both, and like both. For what you are doing, I'd lean more towards the Roubaix. It's just a more versatile bike, all things considered. And it's well executed. If you were racing, or lots of climbing & descending, then the Tarmac might suit better. But general group rides at any level, or alone, the Roubaix is the one you want.

I own a Roubaix and a Diverge. If the Roubaix would take a wider tire (I've only been able to run up to a 32) I'd sell the Diverge and just change tires or wheelsets on the Roubaix when I want to ride gravel. I haven't looked at the web site for S&R numbers but when I put those two bikes side by side (both 58s) they look identical, and I have the seat setup the same on both.

I guess my point is take a look at the Diverge because you can run road tires on it for the road or gravel tires for gravel which is where more and more people are doing their riding.

I am going to disagree with almost everyone and recommend the Tarmac. I have been on some version of a Tarmac since 2007 and it's just plain fantastic to ride! Including on light gravel, and you can run 28s in it according to Specialized. I just feel you give up too much with the relaxed geometry unless you are going to ride primarily gravel, and if you're going to do that you should get a dedicated gravel bike.