I want to shoot cowboy action and I can't help but notice most currently shooting cowboy action are using the shorter barrels. I would love to be able to shoot both and compare.
I've had lots of guns over the years but I've never had real colt single actions. I'm thinking at my age they are not going to get cheaper and if I'm going to ever do it I better soon do it because of my age it's not like I will be able to do it 30 years from now.
I have both a 4 3/4 inch Pietta and a 7.5 inch in .45 Colt. I have extensively chronographed the two using both smokeless and black powder loads. There is remarkably little difference between the two. In Blackpowder loadings, a 35 grain load of compressed 3Fg and a 255 grain bullet gives me 915 fps average out of the 7.5 and about 900 out of the 4 3/4. With Unique loadings of 8.0 grains and the same bullet I was getting more or less the same velocities out of both -- but that was a hot day in the Okanagan. With 40 grains of 3Fg and the same bullet on the same hot day I got around 950 out of the 7.5 inch average and 935 out of the shorter barrel, but interestingly enough the highest velocity out of 10 shots fired -- 5 from each gun -- was a 967 out of the shorter barrel. A compression die helps a lot if you want to make 40 grain loads in 45 Colt and maintain OAL with a 255 grain RNFP bullet. I use SPG or Black Powder Gold (now discontinued) lube on those loads.
I read an article from John Taffin years back where he had reported noticing the same thing in Peacemakers produced by various makers, but had discounted it until it happened to me. Interestingly enough, a friend's 5.5 inch Uberti Peacemaker chronoed about 25 fps slower than my 4 3/4 inch using the same loads. Hmmmm.
My two Piettas came hitting a bit low -- like three inches low and to the left -- like four inches left at 25 yards. By careful filing the top of the front sight I have been able to raise the POI to the 25 yard hold I like (slight 6:00 O'Clock) and by carefully filing the right side of the rather wide front sights I have brought POI back to Dead-On with the 4 3/4 incher and to within an inch of where it would be dandy with the 7.5 incher. A bit more careful filing this summer should leave me with a front sight of about .075 thickness to be dead-on as well.
If I were to buy a new Peacemaker in .45 Colt (and I may well do so, but for down here in Mexico. I am in Salamanca right now as I write this, so I cannot run and photograph my chronograph log for you, you'll have to take my word I'm afraid). I would buy a Pietta in 45 Colt with the color-casehardened frame and the 4 3/4 inch barrel. But that's me. Cimarron sells an Old Model Pietta Frontier in .45 Colt that is identical to these two models, but with the old Black Powder Frame and I think it would be neat to have one here. I will have the Stork that brings the babies bring it for me.
My 7.5 inch Pietta Peacemaker with some Colt-style Mexican Eagle grips I fitted onto it. I bought it from a forum member and he sent it to me with a Colt basepin, which is much better than the factory Pietta part. It generates the velocities that the originals were supposed to generate with the blackpowder loadings of the day, so I think it's "on".
My 4 3/4 inch, which I consider to be my wife's although it's registered to me. Guady and flashy, the little beast really roars with 40 grains of black, and wants to fly out of your hand. Here in Mexico, guns aren't sportin' toys, they're for real use and this little monster would sure do the job.
And the two together on a rug.

I hope my answer gives you food for thought in your decision.