Depends on the situation really. On bosses I tend to prefer max dodge, while on trash I go for the maximum blockvalue + rating.
I don't have any threat problems on bosses, so the hard slams aren't needed.
But, as a starting tank, I think getting that 30% block ( if that's available with your current gear ) might be a good idea, as it will stop alot of damage. It's better to have 30% block if your dodge isn't that high yet.
I could be wrong, but that's how I feel about it.