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.
You can use the E7405A as a basic spectrum analyzer, but you will want to set the resolution bandwidth coupling to SA instead of the default EMI so that they will couple to the span instead of the frequency. Also, with the E7405A you cannot install any of the other optional measurement personalities that you could in the E4407B, nor can you get the EMC Analyzer personality in the E4407B.
We also have the following FAQ:
http://www.home.agilent.com/agilent/editorial.jspx?cc=US&lc=eng&ckey=401800&nid=-35261.536880469&id=401800
I hope this helps,
Mike