Well heres my opinion on your quandry; Take a Slann.
Like someone has already said, magic is incredibly potent in 8th, and the Slanns disciplines make it the finest wizard that any general can field (in my opinion). When your playing any army you have to look at that armies strong points, as a lizzie player you have 3 main ones. First - sallies, second - blowpipes, and third - the best wizard in the game (by far). So make sure you have all of these elements in your army.
In my opinion if you prefer an OB to a slann, play a different army; there are several lists that can produce a similar but superior unit to the OB/Carnosaur. There are NO other lists that can make a caster as devestating as the Slann.
You may want to argue that some armies are built to defend pretty heavily against magic, but they'll have to have spent a fairly hefty amount of points to try and nullify the Slann, and I've not come up against one yet which can hold the Slanns magic off for longer than a few turns.