Having owned the "first generation" version of the Invicta almost from the get-go, I've often asked the same question. I mainly use the Invicta to feed a separate amp (currently a Questyle CMA800R), so I've often wondered what would be gained by moving to the Mirus version. Granted the specs (esp S/N ratio) are a little better in the Mirus, and apparently there have been PCB improvements as well, it would be difficult for me to justify an "upgrade" to an already incredibly good dac, which may or may not yield an easily audible improvement, especially when taking into account all of the other variables - amp, phones, and especially the source material - all of which will probably contribute more to overall SQ than the dac itself. The original Invicta was designed at the outset to be a very high performance dac, and it's worth mentioning that the basic dac chip set (ES9018) and overall circuit design have been retained in the 2nd generation Invicta/Mirus. An additional factor to consider is the company's outstanding policy of providing continuous hardware/software updates, which enable even the first generation units to function optimally and competitively with the 2nd generation versions.
My recommendations (based on personal experience) are: if you already own a 1st or 2nd gen Invicta (with headphone amp), it would probably not be worth upgrading to the Mirus, even if you only plan to use an external amp; however if you plan to purchase a Resonessence model with the intent of only using an external dac, then go for the Mirus -- no point in having the headphone module that you don't use, and you would still have the satisfaction of having THE best Resonessence dac, even the audible improvement may be difficult to hear. If you have even the remotest inclination to use both an internal HP amp and external amp, then get the Invicta model. I could see where this might be the case -- you use the dac "on location" for professional reasons or you want to have a "just in case" backup HP amp.