As someone who plays blue alot, i'll tell you that Rewind, while not utter crap, isn't that good of a counterspell.
The key to remember is that your lands do not untap until after the spell resolves, which means, in a counterwar, or in the case where your opponent follows your counter with multiple instants, you're effectively left with a 4 mana plain counterspell.
On top of that, it's 4 mana, which means that it'll take a blue mage normally 4 turns to bring it out. Against fast decks, that doesn't matter anymore. Don't get me wrong, the Untap land mechanic is really strong, but it's useless alot of times on Rewind.
It's an okay counterspell under normal spellcasting conditions, but sucks when you're playing against instant speed decks.
Dismiss on the other hand, is better than Rewind, because while it is also 4 mana, it can net you a card, and to blue, card advantage is everything. Also, in my opinion, it's performance is alot more consistent then Rewind. Basically, I like it alot more.
In all honesty, I wouldn't more than 2 of either in my deck, but I think that a consistent draw ability is more important than the untap ability.