Hi Bruce. Just one persons opinion here so please take that into account.I started writing a long winded reply with pros and cons etc, etc then thought to myself, just get to the point man ! so you are going to get the readers-digest version below.
I am not sure if you have any interests below ~15 kHz or what kind of antenna(s) you plan to use so it's hard to help you pick something.
I am going to assume you do not have any interests below ~15 kHz and that you will be using some form of active loop, vertical, or e-probe antenna. Having an active antenna with some net positive gain at VLF can offset a receiver with a not so hot front end.
Bottom line, I would get the latest version of the RSPdx which I think now is the RSPdx2. I use my old RSPdx for just about everything and if I could only have one SDR for "SWL" that would be it. My 2nd choice would be the Discovery HF + esp if I were going to use a passive antenna at VLF-LF.
I have an AIRSPY HF Discovery +, a Flex 5000A, RSPdx, RSP1, and probably several others. Oh, I like using a Behringer UMC202HD sound card from 5 Hz to about 70 kHz to study spherics and other natural earth emissions. The discovery has a hot front end below 100 kHz but I do not care for it's "cheap like" little plastic case, tiny size, lack of antenna ports and some missing internal filtering to cut down out-of-band interference etc.
With my active antenna(s) I do not see any difference between the Discovery+ and RSPdx above about ~15 kHz. I do find the RSPdx easier to operate and adjust with programs like SDRConsole. Formal bench MDS tests on the RSPdx confirm very good sensitivity above about 50 kHz.
73, Gedas W8BYA