Assuming that both can have the same options installed (or is that a bad assumption in itself?), what are the differences in what you can do with a E4407B versus a E7405A? Is the E7405A "crippled" in some way to make it only useful for EMC testing, or can it be used as a general purpose spectrum analyzer? Is one of these devices a superset of the other in terms of functionality, or is there some non-overlap between the two? It wasn't obvious to me by reading the specs.