The reason I went for Gaskell is because of his role on the pitch, playing at stand-off. He'll be the one dictating the attacking moves and organising the attack. He also is fairly experienced for a man of his age; he played in the 2011 Grand Final.
I wouldn't have Ferguson as I don't trust his fitness and I haven't really seen enough of him to make a judgement.
O'Brien only really started to prove that he could turn out to be a class player at the back end of the season and we were all impressed in the progress he made in that time. I'm not sure if I'd go for him this time around but definitely a future captain of the club.
I can see why people like Sidlow but he's a prop and isn't going to be on the pitch for the full eighty minutes. I like the captain to be someone who is on for the duration and is always telling his players what he wants throughout the game. This is of course just personal preference.
What we don't see is what these lads are like on the training ground. We don't get to see which players are the most enthusiastic and we don't always get a clear view of their personalities. I'm sure that Jimmy will get it right.