Help me decide: Trek Madone SLR 6 2019 vs. Cervelo S5 2018

A huger percentage of aero bikes will be harsher riding than normal road bikes, if you want the stiffness of an aero bike without the bone jarring painful kidneys then I would go with the Trek even though it's a tad heavier because the Trek has shock isolation features built into the top tube that's adjustable for whatever preference you like, so far no other bike has been able to do that.  If that ride stiffness is a non issue for you then I would go with the Cervelo due to the much lower price.  But remember, this is a bike you will have for a very long time, if after you buy the Cervelo and 3 or so months into riding it you think, "dang this thing sure bangs down roads, wish it was more comfortable", well then it's too late to go back, either that or you'll find yourself not riding it much.  

Another bike to consider that has done something to the CF material that makes the frame that claims it removes 85% of the vibration, that's a big claim, but reviews seem to like it, is the Bianchi Oltre XR4.  At least it has a very nice appearance to it! LOL

I would say test ride them but most shops just have you make circles in a parking lot, you can't tell much in a parking lot. 

Anyway probably not much help but just thoughts.

The Cervelo is a steal (relatively speaking) at that price. It's a very stiff frame (I have one), and with a nice set of wheels for race days/good weather, would be very hard to beat. Perhaps a Di2 upgrade kit. Still cheaper than the Trek.

I bought a hugely discounted S5 a few months back and love it. I could never have afforded to pay full whack, but got full Ultegra r8000 for just under £3k. It's quirky looking (the aero handelbar is not for everyone), it's not lightweight (its not especially heavy though - with pedals, barfly mount, two bottle cages it's just on 8kg), but it's so much fun! I get a buzz out of just owning it let alone riding it. Sure, it's 'old tech' (ooh, you ride rim brakes??), but I've done short rides, longer rides (120km), up hill, down hill, and I - as I said it - love it. If you like the look of it, like the colourway, trust the brand, then it's a heck of a lot of bike for the money.

For the difference get the Cervelo, buy some upgraded wheels, decent tyres and kit. The OCLV 700 series isn't all that special and Ultegra + £250 wheels on a £4.5k bike is a piss take IMHO.

There's a guy near me that sells high end performance bikes from the trade I think, gives very big discounts. Sells through ebay mainly https://www.ebay.co.uk/itm/9000-Parlee-ESX-Di2-Power-Meter-dt-Swiss-knig...

So a mangy two bit Trek or a Parlee with carbon hoops, Di2 and power meter cranks for nearly a grand less (I think he does cash/direct payment with a bit off or part echange)

Go for the Cervelo. Much, much cooler than a Trek.