Here is what
@Caligula gave me about them:
I have parts for both since they are interchangeable. Really which one depends on your level of experience and how much effort you want to put in learning how to properly use and care for your device.
From a technical standpoint, the SR-71 is superior. It has less parts which need to be replaced if they wear out, and its slightly more powerful.
From a user standpoint, the SR-74 is easier to get the hang of and use without overheating your oil or needing to check different things to make sure its operating properly.
They are both great units, however Im not sure if you can even get an SR71 anymore unless its old stock from a store somewhere? I may be wrong. In either case, if you CAN get an SR-71 I say get that and then buy an SR-74 "7.4v white rod". That will give you the best of both worlds without much compromise.
Also be sure to post in the Persei thread and ask a LOT of questions. These arent easy to master but once you do, nothing beats the Hercules carts.