Originally Posted by mstateglfr

For sure- someone with back problems may need a different fit and gravitate to the option with more stack and less reach.
The OP didn't mention any flexibility issues though and the examples I cited simply don't have enough variance in geometry to say one will be noticeably better than the other for a rider with pain or limited mobility. Frankly, neither seems ideal for that.
When compared to options across the categories, a Synapse is pretty aggressive on geometry compared to a Domaine, for example. This is why I advised the OP to ignore the marketing label and focus on actual geometry.

In the OP's case, they want a bike that won't hold them back. That, to me, means no back pain or other limitations. In that case, a Synapse stem being lowered 15mm and adjusted in length to match the stock CAAD setup is absolutely possible. Or if the OP needs a larger frame, then the differences are even smaller(see the geometry screenshot) and even more easily matched.

Simply flipping a 100mm long 6deg stem so it's -6 results in 20mm of bar drop. That alone, a stem flip, more than accounts for the stack height difference between models in the smaller size.

My initial point was to try and show that neither bike is going to hold the OP back and that it's largely a waste to focus on the idea that 'endurance' is slower or more sluggish feeling(since steering and chainstay are either identical or so similar it won't be noticed).