Update: I said screw it and just bought both to see them first hand and I'll just return the other. The 510C already came, the MRO will come next week. Since I didn't get any opinions, which is fine btw- I'll figure it out, I'll just turn this thread into a little review/impressions of each optic on my 9kt (MP5k) in case anyone else is looking for information. I'll try and keep this organized with this in mind. Feel free to ask any questions.
First impressions with the Holosun 510C: I went a bit brain-dead when I ordered it so I accidentally ordered the red version instead of the green. I'm not sure what I'll do if I end up keeping the 510C, most likely return the red and get the green because I feel like a green reticle is superior under most circumstances IMO.
That being said, the red version is still nice, crisp, clear. I have no issue picking up the reticle and acquiring my target quickly. I really like the idea of having the circle and dot (eotech) reticle for something meant for CQB, I think it helps with getting the sight on target a little faster, especially when pinpoint accuracy may not be needed- although if it is, you do still have the dot in the center: I have not truly tested this at all, just my impressions from walking around my apartment doing a little "role-playing." The auto feature of the 510c is not too bad- it will try to use the solar panels to power it first and then supplement with any battery power if needed. In generally, this feature (can be turned off) works well but is not quite bright enough for me, it is close though, but not enough for me to use the auto mode over the manual mode for the time being. I have not tested the shake awake/motion sensor that turns the sight on/off during use or idle time enough to comment on it; you can adjust it to shut off after these increments of inactivity: 10 minutes, 1 hour, 12 hours, or disable the sleep mode altogether.
True first thoughts about the 510C (build/material quality) were quite good, everything seems to be of good quality with no real red flags- I think it is pretty sexy. As mentioned in the OP, I was concerned about the 510C looking too big/bulky and out of place on my mp5k; this was not the case at all, I think it looks fantastic on it and the large FOV is what I was looking for and why I steered away from the aimpoints/clones. and I am glad I did so. The PTR's rail and the integrated quick detach of the 510c work great together and puts it in the perfect placement IMO; this may seem obvious but I'll say it anyways- you cannot use the irons at all when using the 510c, but the QD does allow for it to be removed very quickly. I will say that I wish it came with some kind of cover so it doesn't get banged off anything during transport, but a pretty minor complaint.
I will be going to the range shortly and will put up a range report, but I expect for the 510c to do very well is the first impressions were any indication of its quality.
Side note: Can a mod/similar give me permissions to post images as I think it would be beneficial to my little "review"/opinion. Or is there a way that I can post pictures and I am not aware of it? Regardless, I will work on getting my post count up so I can at least come back and edit these posts with pictures.