I agree they are very different. Seider is taller, rangier (incredible wingspan), more dynamic skater, more creative, and handles the puck like a forward.
Reinbacher is better defensively and has good offensive tools, but Seiders tools are special.
Reinbacher is by far the better skater. Seider may have decent foot speed but Reinbacher is way more mobile. It's that mobility that allows him to make those quick quality passes when there's no time and space.
Seider is better than Reinbacher defensively though. Reinbacher himself says he needs to work on his game against the puck. He knows he's got elite passing and skating but said he needs to hit the gym and work on positioning against the puck. He's over aggressive far too often. Seider used to have a similar problem but I think it was more of a desire to go for big hits or a lack of consistency/concentration than faulty positioning in general.
I think people need to understand that Reinbacher is an offensive defenseman with elite tools and (if anything) defensive flaws and not the other way around. This may not be clear at first sight because he doesn't like to carry the puck or do fancy stuff with it and his preference for making fast plays. However, those passes require skill as well and they lead to offense. He's also very adventureous without the puck, both with his team and the opposition in possession. He's not a defensive defenseman. A defensive minded coach is gonna prefer Seider over Reinbacher, I can tell you that