Casey will be a lot better than Girard in my opinion. Girard was never as sound defensively as Casey, and Casey plays with a great deal more edge and compete to his game. Girard was always going to be a "monitored minute" type guy, but Casey is a guy you can trust on the ice with a one-goal lead in the final minutes against the opposition's top line.
Literally, the only knock against Casey is his size. That's it. With Girard, it was the fact his entire game is tailored to a sort of over reliance on his strengths (skating, puck handling, on-ice vision) while trying to offer sort of a passable facsimile of his weaknesses. Girard is kind of a good version of the prototypical "small, all-offense defenseman".
Casey can be more than that. I literally see him in the mold of a young Brian Rafalski or Kris Letang -- not as a player comparison (please don't assume this, folks), but as an undersized offense-first defender who can also play very well in the myriad aspects of the 200-foot game.
The last thing I want to say about Casey is he's one of the most versatile prospects I've seen in some time. Not only can he play either side defensively in any situation, but he's also been deployed as both a wing and center by both the US-NTDP and U of Michigan, and he's actually a very good forward as well. This is a guy who gives a coach the option of going 11-7 because you can just slot him up to your middle six if Plan A isn't working.
As
@Guttersniped will certainly point out, I'm traditionally very critical of small, offensive d-men and my scouting bread-and-butter has always been projecting defensemen. The fact I'm so high on Seamus Casey is a guarantee of nothing of course, but hopefully it spurs a few people to at least give him another look. He's a heck of a prospect, right now my clear-cut #1 for the Devils.
