I own both a Model 625-6 and a 25-2 and shoot bullseye with them, so you found the right place to talk about this. Dale53 will hopefully show up and add his opinion with ours.
Both guns shoot great, I prefer the balance and feel of the 625-6 with its' Hogue grips and 5 inch barrel. I have a Weigand scope mount on mine and have mounted the fantastic Aimpoint Micro R-1 red dot on it. I had a wonderful action job done and the trigger breaks right at 2.7 pounds. I have to use Federal primers with this gun, the other brands just won't go off. I have shot Master Class scores with this amazing revolver.
The grips are terrific. They feel different from the wood target grips on the Model 25. For me, they are easier to maintain control of the gun during rapid fire shooting.
The Model 25 is slightly heavier than the 625-6 but the difference is not noticeable.
Speaking of moon clips - for bullseye shooting you can use the nylon moonclips. Do a search for them online and you will quickly find them. They require no tools to load and unload and bullseye shooting is done under very controlled conditions so you don't have to worry about rounds falling out of the moonclips. They make life easier. I buy about 30 of them and they last me 4 years, then it's time to buy new ones.
The Smith Model 25-2 is also in .45 ACP/.45 AR. You mentioned the .45 Colt - which is the Model 25-5. You cannot shoot .45 ACP ammo in the .45 Colt gun as is and I don't know who could alter the gun to allow you do so. So, if you have not yet bought a .45 Colt, do not do so if your intention is to shoot Bullseye. Get it for just plain fun shooting.
The Model 25-2 is also a wonderful gun. I do not shoot quite as well with it as with the 625-6 but it just may be me. My gun was made in 1961 so I had to have it drilled and tapped for the Weigand mount and I mounted another Aimpoint Micro on this gun too.
I also have a Ruger Blackhawk with .45 ACP cylinder. What loads have you found that shoot well in yours? I'm still looking!
For the Model 625-6, try all the usual target powders - Bullseye, 231, Clays, WST, 700X, Titegroup, etc..... Start with 200 grain H&G 68-style cast bullets shooting 4.0 to 4.2 grains of the mentioned powders (except Winchester 231, use about 4.6 to 5.0 grains of that powder). You will find something your gun likes. My gun shoots 10-shot groups of 2.5 inches at 50 years off sand bags. Have fun and stay safe!!!
