I am by no means an expert on such things but I would like to throw in my 'four penny worth' by saying I opted for a steel box...not Spanish built granted...but steel all the same.
My reason for this is that i only use it very infrequently and i was told that you need to warm up a cast iron box slowly in case it cracks whereas a steel one you just chuck some tinder on and flame away!
The other difference Im told is the steel offers 'instant heat' whilst the cast one retains heat.