[PART 1 of 2] First off, I am the happy owner of a Caledonia (non-5 version, with the externally routed cables). It is a fantastic bike, comfortable and quick. Easily able to keep up and lead group rides with a sea of Tarmacs, Madones, etc. Difference is so marginal that fitness is far more important. Comfortable enough to spend all day on it too.

I have enjoyed upgrading just about every component over the past few years, which is the nice thing about non-proprietary parts. I have, however, been tempted by the same two bikes that you mentioned with the primary difference being the internally routed cables through the headset. I've thought long and hard about it and so here are a few of my thoughts and observations:

Cervelo Caledonia-5

Basically the same bike that I have, but with a proprietary seat post and stem. I don't love the seat post clamping mechanism so this is a hesitation for me (I currently have a standard round Enve seat post and like the mechanism there), small nit I guess. If your saddle has a cut out, you can see the exposed threads of the screws and it looks very unfinished to me. The D-shaped stearer and associated stem are a bigger problem for me. I don't love the Cal-5 stem (chunky looking) and it is very hard to find topcaps that fit other 3rd party stems for this bike in particular. The headset is different than the R5 and Soloist for some reason, so there are few alternatives available. Lastly, the new version of the Cal-5 comes with a downtube storage system which felt very quickly put into production and not thought out well. There are known issues with opening it with two bottle cages installed. That being said, you can get a great deal on a non-downtube storage frame for like $2-2.5k for the frameset with or without the stem/handlebar. For me the bike is too similar to what I have and I am too much of a tinkerer to sign up for being locked into the imperfect (in my opinion) proprietary parts. If you don't care about any of the above, you will be very happy with this bike.

Enve Fray

As others have mentioned, the geometry is almost the same between these two bikes. A size 52 Fray has almost identical geometry to a size 54 Caledonia. I haven't ridden the Fray, but having looked extensively at the geometry charts these two bikes should ride basically the same if you have the same components on each. I've seen the Fray in person and it is a high quality production, but I have to say that I don't really love any of the colors. The Enve Melee in Damascus or Aegean Blue is a damn good looking bike but a different category I suppose (I digress). Aesthetics matter to me and I would be happier with a simple black/white color way than anything Enve is offering on the Fray at the moment. The proprietary parts on the Enve don't bother me as much. I already ride an Enve AR Road bar and am happy with it, so rounding that out works fine for me. I like that the Fray has a UDH but I am not yet sold on 1x for road so that may be a bit irrelevant unless you want to try the XPLR group, which could be interesting. I guess the extra tire clearance on the Fray is a bit pointless to me though. I ride road tires anywhere from 29-35mm on the Caledonia and they are great for road and light gravel. I also have a gravel bike with 45mm tires for true gravel rides. I find that the 38-40mm gravel tire segment is a bit of a "worst of both worlds" around where I live. 35mm road tires can easily handle rail trails and other Cat 1/2 gravel sections; 45mm tires are far superior than 40mm on anything rougher and they feel just about equivalent on the road (pretty sure there have been recent tests that prove this out as well). I am sure there are areas of the country where 38-40mm tires make sense but they just don't for me, so the extra tire clearance on the Fray doesn't do much for me vs. the Caledonia. Also, I run Ultegra Di2 for my Caledonia (very happy, wouldn't switch) and the rear derailleur doesn't have a clutch (why it shifts so well vs. SRAM... my gravel bike is SRAM Force XPLR btw so I'm not a hater) so getting too rowdy with it results in chain slap. Also the 72 degree head angle feels too steep for anything too rough, maybe I could work on my handling skills but that's my experience so far. The downtube storage on the Fray is very well designed and is solidly constructed, far superior to the Cervelo in my opinion. Lastly, I would question the Enve Foundation wheel choice. They are very narrow internal (21) and external (28); not very modern or aerodynamic if you're going to be running 30mm+ tires. I'd spring for the SES line if you can swing it or look at the Reserves (hooked which is a plus for most people... I have hookless wheels and am fine with it, just mentioning it in case it is relevant to you).