Just how different are Yamato and Kosaburou to Cassidy and Butch?
Quite a lot, actually.

                      First off, Yamato and Musashi *always* hated each other. None of this "former
                      friends" stuff.
                      Yamato kept on beating Musashi at everything during their earlier Rocket days, much
                      to the discontent of Musashi. (Think Shigeru Vs. Satoshi, and how Sigeru is always
                      many, many steps ahead of Satoshi. Exact same thing here. I have *NO* idea
                      *WHY* the dub decided to cut out the rivals thing. Are villains not supposed to
                      have rivals?)

                      Second, Yamato and Kosaburou are a *lot* nastier. The dub takes away a very
                      noticeable edge off them, leaving them watered down. (They also do this to the boss,
                      btw) I think a comparison that best illustrates this is:
                      Musashi/Jessie & Koujiro/James are begging Yamato/Cassidy & Kosaburou/Butch
                      to let them go without telling the boss. (Breeding center ep.)
                      Cassidy says: "What's it worth to you?" (As if she'll be happy with a simple bribe)
                      Yamato says: "What are we going to do?" in an almost singsong voice, as if she
                      knows *exactly* what she's going to do, and it involves causing Musashi the most
                      amount of pain and suffering possible, most likely telling the boss exactly how much
                      Musashi interfered with their operation.
                      Simply put, while Cassidy sounds and acts tough, only Yamato has the potential to
                      be spine-chillingly nasty. (And yes, this *definately* applies to Kosaburou vs. Butch,
                      but I'm too tired to dig up an example of Kosaburou being a lot nastier than Butch.)

                      Third, in the Japanese version, it's flat out said that Yamato and Kosaburou are
                      some of the best Rockets on the team, miles and miles (and miles....) ahead of
                      Musashi and Kojirou. The dub only hints at that, and not very well, either.

                      Fourth, Kosaburou is the (very) smart one, always thinking up the plans.  There is
                      no mention of Butch's intelligence, on the other hand, or even if he has any.  (Another
                      one of those "WHY did they decide to cut this for the dub?")

                      Fifth, It's never explained how Yamato and Kosaburou get out of jail. Cassidy and
                      Butch get bailed out by the boss, but such an explanation is not found in the
                      Japanese version.

                      Sixth, in the Japanese version of the Drowzee/Sleep episode, there's this big motif
                      of "being the better villains" and variants of that theme. Yamato and Kosaburou
                      claim to be the better villains, as they are much more successful and nastier, yet
                      Musashi and Kojirou claim they are the "better" villains, since they are acting better
                      in the eyes of the law. Since this clever motif requires some thought, It's completely
                      cut out by the dub. (Can you tell how much I don't like the dub?)

                      Seventh, Yamato and Kosaburou are around 22-23, while in the dub, it's hinted that
                      they're in their late teens, 17-18.

                      Eighth, Kosaburou's Japanese voice is MUCH MUCH better than the dub. I say
                      this because instead of getting that infamous raspy voice of Butch, Kosaburou as a
                      much (much) nicer, deeper voice. I'd even use the word suave to describe it.
                      Yamato's voice is also better.

                      Ninth, that whole scene of Yamato and Kosaburou letting in Musashi and Kojirou
                      in the Drowzee/Sleep episode is completely different. Instead of Cassidy and Butch
                      being foolishly tricked, like it's portrayed in the dub, the Japanese version has
                      Musashi and Kojirou give very good reasons why they should be let in. (Such as,
                      they truly do fully surrender to them, before stabbing them in the back..) I'd say the
                      dub changed this to further reduce the perceived threat of Yamato and Kosaburou.

                      Tenth: "We believe in love power, that's because we love power!" (dub)
                      vs.
                      "If love is everything in the world, what is precious money?" (Japanese)
                      'Nuff said.

                      I could go on for hours about the minute (and not so minute) differences between
                      the different versions, but I think I've given a pretty good outline of just how badly
                      the dub treated Yamato and Kosaburou.  There's a reason why I prefer the Japanese
                      version.