Sure I can provide some impressions.
They're both outstanding products, and I'd love to own them both, let's get that out of the way.
I have the Thummim on order, so treat that as bias if you want to, but as I say, I'd love to have them both.
With the Erlkonig, you get an audiophile sound that's just refined to near perfection. When I was deciding on this or the Thum, what made me choose the MMR was that it's so different from anything in my collection, and so different from anything in the IEM world.
My reasoning was that I have the U18t - and the Erlkonig does pretty much everything it does, but better. Very clean sound, but with more musicality. The Erlkonig just organises everything so well, it's a marvel - tight, deep and focused bass, a midrange as accurate as almost anything out there, beautiful highs...in a nutshell it's an audiophile's dream. But when I was feeding back to some friends who were interested in the comparison, I said something along the lines of "the Erlkonig will wow and impress audiophiles - the Thummim will wow and impress almost everyone".
The Thummim has a larger-than-life sound - almost hyper-real but without crossing over into a zone that it becomes so unreal that it doesn't sound right.
It's got a very slightly muddier, but more textured bass, but I wouldn't say it dominates the sound. As has been mentioned many times, it has a W shaped tuning, so everything is available and detectable, but outright resolution takes a slight step back in favour of fun musicality. I just remember wanting to listen to every album in my collection to see how much fun it would be, whereas with the Erlkonig, I just want to listen to it all to hear what sounds I've been missing all these years.
At the end of the day, I already own the U18t which I'd be happy to upgrade one day if i found an Erl at a reasonable price. But for now, if I want a nice, clean sound, I can whip out the U18t and listen away.
But I own (and know) nothing that sounds anything like the Thummim. The soundstage truly has to be heard to be believed, it's phenomenal. And at the end of the day, the only time I listen critically is when I'm auditioning/demo-ing stuff. The rest of the time, I want that fun, so I went with the Thummim.
edit to say: happy to provide more specific opinions if I can, so let me know if you have any questions