I would go with the CAAD Ultegra...UNLESS you do a lot of riding in the rain, or plan to upgrade the wheels later to carbon fiber, or weigh over 280 pounds, then go with the 105.
On dry pavement there is no discernable stopping ability between disk and rim brakes because the real thing with braking is all about the tire's adhesion to the pavement, not about disk or rim...UNLESS, you have carbon fiber rims then rim brakes are a major fail and you must use disk brakes. Rim brakes have been used in professional racing far longer than disk brakes have, and pros racing down the Alps at 70 mph didn't all die at the end of the descent and are buried in mass graves at ends of sharp turns! It's crazy some of the stuff I hear about disk vs Rim brakes these days, and we have marketing forces to thank for creating hysteria.
Another real case for disk brakes is mountain bike riding, and you are actually riding on mountain paths all the time, disk will work better in those cases, but I use to also ride a mountain bike on rough dirt trials up in the high Sierras and other mountainous areas and never had a problem with rim brakes then either. Disk brakes were originally made for mountain bike riding, they then discovered that CF rims on road bikes had major issues with rim brakes, from not stopping fast to delamination of the rim from the heat of the brake pads, so disk brakes found a new home and that was for CF rims on road bikes, and now they want all of us to use them even on aluminum rims which is just nonsense.
On my new touring bike the bike came with disk brakes, and all the bikes I looked at that fit my needs also all came with disk brakes, so I had no choice. My old touring bike came with cantilever rim brakes, and I loaded up that bike with the same weight as I do with the new one, and other then a easier smoother feel to the brake lever when applying the brakes of the disk brakes they stop the same.
About 4 or so years ago because of a discussion on a forum about disk brakes were superior, I tested my 13 Lynskey with 105 brakes against a friend who had just bought a new bike with Ultegra hydraulic disk brakes, the combination of our bikes and body weight were within 2 pounds of each other, he had better a and wider Conti 4000s tires and I had narrower Vittoria Rubino tires, we did 3 runs of each, the first set at 15, the second at 20, and the third at 25 mph, we had to stay seated on the seat and not move our rears off the seat and over the rear tire. The first 9 stops we both stopped within a foot of each other, sometimes I was a bit shorter and sometimes he was, we attributed that to reaction time. But the last set something weird happened, the first stop from 25 was like the others, the second stop I stopped about 2 feet shorter, but the third stop I stopped just a bit more than 3 feet shorter! Why? because the disk brakes from repeated stopping were fading, they got so hot my friend blistered his finger when he touched the front rotor, then he touched my front rim, while hot it didn't burn his finger. See the larger rim area cools much better than the smaller area of the rotor, so all that heat had nowhere to go and it made his brakes fade. Of course Shimano is trying to answer that problem with a new rotor that is supposed to cool a lot better.
If you chose Ultegra you cannot later switch to disk brakes on that bike without incurring a major expense.
I use to race up to cat 3, in the mountains of S California mostly, and never wished for better brakes. You can improve the Ultegra brakes by replacing the stock black Shimano pads with Kool Stop pads, either all Salmon colored or dual compound pad that is 2/3rds salmon and 1/3rd black, personally if you can get the all Salmon colored pad to fit the Ultegra brakes go with those. The salmon brake pads are superior in the rain over the black compounds, but I they work darn good in the dry as well, so I don't see any advantage with the dual compound. I had original Shimano black pads that my Lynskey came with, and they were crappy pads, not only in the rain but they felt mushy even when dry, I replaced them fast with Kool Stop Salmon pads and they feel much better. Also the salmon colored pads last seemingly forever, instead of just lasting 1,200 miles or so that the black ones do, those Salmon pads could last 15,000 miles!
Anyway, personally I would go with the all Ultegra because it's cheaper! My Lynskey is all 105 except the rear derailleur is Ultegra, nothing wrong with 105, it's a great groupset, the Ultegra rear derailleur is built to last a bit longer. I opted for the Ultegra rear derailleur due to it shifting a bit smoother, the rest I didn't care about because the two groups felt so close to the same. So money talks, but in your case the Ultegra is cheaper, so for me money talks, I would get the Ultegra since the brakes are not going to stop you any faster unless you ride in the rain a lot.