If you are going from R3 to a more racing oriented bike, then of the two would be Tarmac, but even that has a big stack.

I looked at both (and a couple other bikes) at the turn of this year.
I think the Tarmac as the previous poster say is lower (less high!
I'm 6ft , inflexible etc so a mega low bike wasn't what I wanted).
I got the Tarmac. (A size 58).

Think the Roubaix may take bigger tyres (certianly fitted with larger as stock off the showroom).
The Tarmac is listed as OK for 30mm tyres. It comes with 26mm I'm reasonably confident it would go to 32... but not needing them myself so not tried).
Think the roubaix comes with 28mm as standard.

What swayed me to the Tarmac ?
A smidge lighter (not really an issue given my excess lard, but still) but mainly that its got 100% standard off the shelf parts so no obsolescence in 3 years time e.g. standard stem (vs spesh stem specifically for the Roubaix Future Shock, standard headset, no stooopid arse about face seatpost that some other brands are using (Trek on the Emonda as an example of an arse about face seat post), no inbuilt shock in the headtube to go wrong, etc.

All I've swapped is the stem (to go shorter - the standard 110mm stem on the 58 is for knuckle dragging apes) and the saddle (to a Cobb FlowMax which is my go-to).
Sold the old bits on ebay no problem.

Both the Tarmac and Roubaix come with tubeless ready wheels. But the stock Specialized Pro tyres are NOT tubeless (grrrrr). Decided I'd run those tubed until they are worn out then go tubeless.

You may find good deals for the current Tarmac SL6 as the new SL7 will be released in the coming weeks...

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I may not be much help here, from a technical standpoint, but I own both the Roubaix and the Tarmac. I loved my Roubaix but purchased a Tarmac when I decided I wanted a road bike with disc brakes. Now that I've ridden many miles on both, my Roubaix sits on my trainer and my Tarmac is the one collecting the endless miles. It's so much fun to ride. Smooth and fast feeling.

Pure road bike & not racing,use case says Roubaix.

As someone who just bought the Roubaix and used to ride then Venge:

If longer legged than top body go tarmac. The 2020 models are all pretty aero, tarmac should corner a bit better.

have owned both, sold my tarmac when I got my roubaix, roubaix is slightly heavier but slightly more aero than the tarmac, it's WAY more comfortable to ride (future shock and pave seatpost combined with bigger tires), takes bigger tires (33s will fit easy). Standard model has a lot of stack, you can get the team version which has a lot less but tbh, I'm perfectly happy with the regular one. I ride some fairly bumpy roads and it's been great. Common misconception is that the future shock wallows if you get out of the saddle or sprint, which I have not found at all, if I'm on smooth roads I'll turn the FS to the 'off' position (which isnt all the way off), same if I'm climbing pretty smooth roads out of the saddle, other than that I leave it all the way open.

What did you end up doing?

Not sure what he did but I just bought a 2020 Roubaix expert. Hope to be riding it next week. Looking forward to putting my TT bike away until COVID is gone and just getting out and enjoying a lot of solo rides.

OP here , still have not decided but it will be a 2021 model unless there is no spec change .
Interested to know how the Roubaix works for you .

When are the 2021s due out? Soon?

This was ultimately what led me to get the Tarmac as well. I went in the store looking to grab the base version roubaix (based on color and price and was simply going to upgrade parts to 105 or ultegra and sell the tiagra stuff) but walked out the door with the Tarmac. I figured that with the shock system it was something I was worried that would go wrong. Since I drove about 1 hour 15 mins+ into DC, I didn't want to have to deal with any issues should they come up. Closest store is 1 hour at least from me. And, as I said in another post, these bikes excite me. The trek Domane was nice, but not like this. I love the frame styling of the specialized bikes and am glad I took the drive!

I also felt like there was not too much difference in the ride feel between the two bikes (roubaix and tarmac).

How are you liking the Roubaix? Deciding between the SL7 and Tarmac as we speak.

Thanks!