Endurance or Aero Bike

I bought a Giant Propel at the end of April, although only at half your budget. I absolutely love it, to my eyes it looks cutting edge in a way that non-aero bikes don't any more. It also isn't black/dark grey/stealthy.

I Everested on it at the end of May, 230 miles in 27 hours so I think it has enough comfort, although that was such a one off event for me it may not be representative. I've also done the Dartmoor Classic on it which is a hard but traditional sportive.

At the time I demo'd a Fuji Transonic, which looked slightly more cutting edge, but the ride seemed noticeably harsher than the Propel. Although the purchase price was similar, the spec wasn't quite as good.

I also demo'd a Cannondale Supersix which was lovely, but didn't excite me as much as the Propel. It may be a more comfy option for you and not as upright as the Synapse?

I found Evans were good for demos - you have to book them, and pay a deposit, but you can choose pretty much any bike at any spec level you want. A lot of shops only have the top models to demo (I also demo'd a 2.5k Propel that blew me away) but with your budget that shouldn't be an issue.

I'd try as many as you can, I'm not convinced the differences between a sportive spec and an aero spec are that huge. The biggest drag on a bike is you, so it may be better to work on your position, and your wardrobe!

Pick something that gets you excited to be going out for a ride, that makes you think I'll just do another 10 miles when you are almost home.

Biggest wind resistance comes from the rider, so in addition to shaped tubes etc on an aero bike they will generally have an aggressive riding position, ie face down ass up, otherwise the improvements on the bike will count for little. Make sure you're comfortable on in that position before laying down your hard earned. Endurance bikes will be a fair bit more relaxed and upright. If you have really good flexibility then maybe not a problem.

They are two completely different bikes with completely different fit.
You say you are coming from a hybrid which will have a fairly upright position so the endurance bike will feel like less of a shock in terms of fit. The aero bike will have a shorter head tube than the endurance bike so the bars will be lower and need more flexibility to ride comfortably.
With £3.5k I would be tempted to get a decent first road bike (£1000) and adapt to its position over time. Then after a few months /year depending on how the bike fits and your level of flexibility then buy your nice bike Dependant on fit.
You could then either keep the first (and have two ) or sell it and spend the money on kit or upgrades etc.
Long story short pick which one will fit you better, however this isn't easy as you can't know this without having ridden a proper drop bar road bike.

I ride a semi-aero bike (NeilPryde Nazare) and have done plenty of long rides on it. I've also ridden a Giant Propel and have done long rides. So aero bikes can be comfortable, but I come from a background of riding road bikes. Coming from a hybrid it might be a bit of a shock to the system. Ideally you need to go have a ride on these bikes, maybe contact your local cycling club they might be able to help out. Certainly I've raced on a Defy and done well in the past.

If I was you I'd be tempted to go down the middle route and get a road bike in the style of a Giant TCR or similar.