In my opinion, they're very different given there are only two of them. Obviously the positions don't mean much at the start of the season because you're comparing 30 players v 30 players and if you have a cheap ruck you'll spend the cash in the other position and vice versa. But, by the end of the season the goal is to have the 30 best players or close to it. In bigger lines like Midfield and Fwd/Def, you have six players to fill that slot and it might be that block of three or four weeks, the fifth ranked defender goes on a tear, whereas another block the seventh best defender does and over time the six players you have sort of level out and it doesn't matter if you had exactly this six or exactly that six. In Ruck, there is only two so there's a lot less diversity. If there are a couple of guys that are above the rest, they'll consistenly put points on your replacement and you'll slip back. It wouldn't be the case if there weren't two clear rucks and there were, instead, a handful who could all score 120+ for periods, but over the last three or four years Grundy and Gawn have proven they're the best and it seems that will continue.
Plus you have less options as less price points to replace them with. There's only like 40-50 players with ruck status and only probably 25-30 playing each week, so if your guy is injured, you might not be able to find a good suitable replacement at the price point you need. Whereas there are hundreds of forwards, mids and defenders making it easier to find a replacement at that position if you do get an injury.
