I have one with a 7.5" barrel and enjoy it alot. I also used to have a 4" GP also but I sold it. It was a heck of a shooter, but I prefer the .44 mag. I have never shot a 5.5" blackhawk but my brother has a redhawk with a 5.5" barrel and it is also a great shooter. Either way you should be happy with the blackhawk. Barrel length really depends on if you are going to use it more for hunting or packing.
later,
Adam