I'm not sold on either as a pet, although I may revise that opinion.
My early feeling is that Crocodile is going to suffer the same as Bridge Troll - a great card that becomes overcosted SBPwise because of his schools. I feel multischool cards (at least without an intended mage with all required schools) need to be undercosted in mana to make up for the fact they will overcost in SBP. Croc is a L4 creature that I would probably use if I could have him for 4SBP (certainly as a control card in Nature books, I'm less convinced in Siren), but I don't think I want at 6SBP...
Crab has the same problem, although a little less severe (at least for Siren). I also feel he needs an extra dice on his base attack, and/or maybe +2 Life...
Of the two, I favour Crab more. As a control player at heart, I'm very interested to play with both, although I fear them both being more Giant Wolf Spiders - great ideas, but overcosted. Unfortunately things like Grizzly are so cost effective, that the best control method is often kill it quickly with a super-efficient creature!