The number that follows the IG indicates model and the higher the number, the better the model. So the IG 30 model isn't as nice a unit as the IG 90, which will last longer and be easier to clean. They should however work the same when brand new.
I have the IG90. Got it on sale for $18 at Performance. Can't say it's that good ... it rusted after my last muddin' session, even though the chain's supposed to be cadmium plated? :confused: Otherwise a very nice chain. I have almost 500 miles on it and it still works like new.