OK, one way to look at it--if you buy either the M3 or M1 the price to add a network card is $1K. It's identical to what's in your M5, without the case.
The entire "Rendu" line is about galvanic isolation and converting the signal coming in to an optical one, which (as opposed to so many things in this hobby) creates a difference you can actually hear.
I have an Optical Rendu in my house with the optical module. It's better than anything else I've had in my system which includes my Innuos Zenith MkIII and the Bricasti M3 that I demoed. The dealer I know who sells the Ultimate Rendu claims it's the best streaming option that he has heard--better than the Innuos Statement, better than the new EMM Labs NS1 Streamer, better than anything he has heard (and he carries lots of $$$ streamers and DACS). Maybe the Grimm beats it, but neither of us has ever heard it.
If you get a chance to demo against the Bricasti I'll bet you will find you are hearing a lot more out of the Ultimate Rendu.