The best carry length barrel will depend more on your intended use. As to the best length for velocity the loads used can be talored to obtain the best for said barrel length. Hope this makes sence.
I have a Raging Bull in 454 with the 8 3/8" barrel. It is long for sure and I have to carry it in a cross draw fashion. This isn't a problem for me as I favor this way when hunting anyway. My other revolvers for the most part sport 7.5" barrels and I can go either way with them.
If your intending to handload for yours which I suggest unless your independently wealthy, a little hiomework, and right the powder choices can bring out the best velocity for just about any length within reason. However for the sake of argument, I would not suggest less than a 6" unless this will be solely for up close and personal last ditch response to something looking to eat you.
For comparrison sake you can generally use around 50 - 75fps velocity loss or gain per inch of barrel. This will however vary somewhat depending on the powder and side arm, but it is close enough to get you in the ball park. If you go to the Freedom Arms site and look at the bottom of the page you sill see a link to their loading data. This also gives some insite to the velocities capable with a 7.5" barrel and you could add or subtract from there.
In my Tarus, I am running a 265gr Cast Performance WFNGC to around 1850fps. Trust me when I say this load rocks, and is just getting into the territory capable of this round. I have not wanted for penetration and in fact shot a feral hog of around 150 or so pounds, with it yesterday hitting it in the squeeler and exiting just under the tail. Needless to say I got adequate penetration. I shot another one of the same pack and around 250 pounds with the impact hitting around an inch behind the end of it's nose and it exited about two thirds back of the offside ribcage, taking out pretty much everything in a 2" path along the way. These shots were made offhand and were around 50 yd each. The accuracy of this particular load is awesome with groups from a rest resulting in multiple clover leaves at 50 yds, which is about as good as it gets.
I do however shoot revolvers quite a bit and at ranges longer than most. To me I am just getting started at 30 - 50 yds with hunting ranges being out to 100yds easily. I know my loads and how they shoot and practice offhand more often than not. Simply using the rest to verify sight in or develope loads.
IF you can afford it the FA in 7.5" would be my first choice, however I was presented mine as a present, so I couldn't complain. I can say that after shooting both, the Tarus I have is a much heavier handgun and the ported barrel keep the muzzle down nicely, however it also adds to the noise level some over the FA. The FA however, is tops in fit and finish, and built to last. The design of the grip takes out a lot of the punch and they recoil nicely even with top end loads. I do however suggest you try and shoot one before buying, as they are not for the average shooter. The recoil from either is very intimidating even for some seasoned shooters. Winchester loads a 250gr load to 1350 which is very mild, and easily found, however it will not give you a clue as to the true power of this round. You can shoot lower end loads for practice however if your going to only shoot the lower loads you might be better suited going with a quality 45 Colt.
Hope this helps some, and not trying to start up anything or change your mind about getting one. Just pointing out some of the pro's and con's I have seen through the years. They are a big investment and a big step up from some of the other magnums out there. THey are however a blast to shoot and will easily with the right bullets take down just about anything.
Good Luck,
