Replacement for S-Works Tarmac SL ?

I've got a 2007 S-Works Tarmac SL that has suffered a cracked downtube so I'm in the market for a replacement frameset. I love my Tarmac so the obvious replacement is a current version S-Works Tarmac SL4 which I may be able to get at a discount on the Specialized crash replacement program. First question is how does the ride quality of the two frames compare ? The frame has got stiffer over the generations but does this mean a harsher ride ? I mostly ride 100km+ weekend spins around the Surrey hills and do a trip to the Alps in the summer (sportives is my thing, not racing) and found the SL to be comfortable on long rides, but worry that the SL4 may not be. Second question is what other frames should I be looking at around the £1500 mark ? I should add that before the Tarmac I had a Roubaix but the more upright geometry didn't suit me at all.
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You can definitely tell they've a similar heritage, the ride is very "familiar". The SL4 is so much stiffer, accelerating and climbing better, but also lots smoother too. Rough roads no longer feel so rough. The handling of the SL4 far sharper.
Specialized Roubaix SL3 Expert 2012, Cannondale CAAD5,
Marin Mount Vision (1997), Edinburgh Country tourer, 3 cats!