Good point, I probably should have mentioned the load event in my original reply. It is a viable alternative to the callback but I'm not sure I'd call it 'a matter of personal preference'. That implies that they're effectively interchangeable but in many cases there's only one that will give the desired results. Difficult to say which from the OP's question.
In most of my apps I don't bother detecting load errors on individual stores, I just have a catch-all error handler on Ext.Ajax instead. It depends what the errors signify and how you want to handle them.