I think you should look at the actual geometry of a CAAD12 vs CAAD13 and decide which you want instead of viewing the CAAD12 as 'better' because it has more aggressive geometry.
I see this sort of thing a lot- Bike A is more relaxed than Bike B so Bike B is better for fast/hard riding. Meanwhile, Bike A is actually more aggressive than 70% of the segment's offerings and that isnt even recognized or understood.

Yes the CAAD12 has a lower stack height and longer reach than a CAAD13. Meanwhile, the CAAD13 has the same chainstay length, same wheelbase length, and same BB drop.
In comparing the largest sizes, the stack height is 9mm different. Just remove a 10mm spacer from the CAAD13 and you now have the same effective use height as a CAAD12. And the CAAD12 is 5mm shorter in reach. But without knowing what handlebar shape and measurements you like along with shifter style you will use, the effective use length is unknown. 5mm can easily be negated with what you select for bars/stem/shifters.

To me, the only difference on paper that will really be felt and cant easily be accounted for and evened out is the trail number. The CAAD12 has 54mm of trail and the CAAD13 has 58mm of trail. That may be something you would notice and may be something you would care about. Or, you may actually like 58mm of trail. See- the number isnt as aggressive, but that doesnt mean it is inherently 'bad'. 58mm of trail when combined with a 408mm chainstay is still quite nimble.


The CAAD13 was advertised as more aero than the CAAD12. How much?...no idea, I just remember it was advertised as more aero.
Two people can ride the same bike and come away with different views. Once can say 'it corners on rails' and the other can say 'steering is too twitchy'. Same bike, same corners, different views. Figure out what you like for geometry on a bike vs buying a bike based on geometry to try out.
One more thing- are these disc or rim brake frames? If disc, the 12 has QR and the 13 has TA. That is a deal killer for some people. No idea if you care or not, but I just mention it so you are aware in case you care.





TLDR-
- A CAAD13 can be set up identically to a CAAD12 in cockpit fit with a couple of incredibly simple and common adjustments to the spacer height and stem/bars.
- They have identical chainstay and wheelbase measurements which both impact how lively a bike feels.
- The steering trail is slightly quicker/twitchier on the CAAD12, but the CAAD13 trail doesnt make it some slow steering pig.
- If these are disc frames, figure out if you care about QR vs TA.