I am sure you will get a response from Dr Bob (Thataway) - he has owned 22 and 25 C-Dorys, as well as a TomCat. We owned a 25 Cruiser and traveled with friends who owned a TomCat. While the cabin on the CD-25 and the TomCat look pretty similar on a walk-through, they are very different boats (a monohull vs a catamaran). The TomCat is designed to cruise at a faster speed (30+ knots), while the 25 can get great fuel mileage at displacement speed, but can cruise in the 20 to 25 knot range as the high cruise. As you'd expect, the ride is different between these two. At speed, the TomCat will get a smoother ride in light chop than the 25. The TomCat will be more stable at the dock as you step aboard since it is a catamaran. The TomCat will be heavier to tow (our CD-25 was 8,700 pounds on the trailer, our friends' TomCat was over 10,000 pounds), so you many want to consider a bigger tow vehicle (we towed with a 3/4 ton diesel GMC pickup, our friends towed with an F-450 diesel Ford) The cockpit on the TomCat is bigger, with the engines mounted behind the hull rather than attached to the transom. The berth on the TomCat is much larger and rectangular vs the V-berth on the CD-25.
One thing that I preferred with our 25 was the quiet at anchor. The TomCat has a relatively low bridge deck, so you will have some "slapping" right under the berth. At slow speeds in chop, you may get some "sneezing" with the TomCat, where it will push water forward and up from that low bridge deck. But, push the throttles forward and the TomCat shines. Another thing was how low our boat sat on the trailer, with the hull sitting between the tires, rather than the TomCat which sits above the tires - it makes the CD-25 easier to launch/retrieve. The TomCat looks, and is, much larger (height and length) on the trailer.
The CD-25 can be had with a single or twin engines; the TomCat will always have twins. I'm sure you'll get a lot more input from other owners. Both are great, but different from each other.