Di2 10spd a very expensive dead end to go down.
Tracing a replacement 10spd Di2 RD will not be fun (or cheap!). 6870 basically combined the robustness of the 10spd wiring loom with the mechanics of Dura Ace.
For your budget, I’d go with 6800 and the CAAD12. I’m pleased with the manual 11spd (have both 10 and 11 Ultegra in three flavours) and it does offer extra gearing options once you swap rear cassette.
Both frames are superb. Will be little to tell them apart.
One last point – are you sure about sizing? Cannondales come up pretty large and I (and others) ride a size down (54 instead of a 56) with a longer stem for this reason. Have you tried either for size?