At different times I've owned an XCR-M, and 3 different M305s.
My experiences with the differences:
1. I owned an M305 18", bog-standard and sold it. That M305 worked, but it had poor accuracy, around >6 MOA.
2. Then I bought 22" M305 in a Sage EBR chassis from another website that I will NEVER go to again (firearmscanada.ca, they're infested with scammers don't buy anything there). The Sage chassis didn't fit the trigger group properly, and so it failed the test-after-assembly. Had I shot it I would have gotten slam-fires. The seller went no contact after that, I wasn't happy. I sold that to a guy on the EE here at a significant loss because I was completely up front about the technical problem, he was cool - he just wanted the stock.
3. So that partially financed the XCR. The XCR was problematic at first with stoppages, but it got worked in after about 40 rounds, but it was a lot better in accuracy with the heavy barrel, 2-3 MOA.
4. But eventually I decided I missed the M305, so I got a third one - a SOCOM 18. I put that in a wood stock, and put a scout rail on it. It looked gorgeous. Took it out, and the accuracy sucked. AGAIN. >6 MOA.
...I was constantly disappointed with the accuracy of the M305. I know it's not my shooting, because I can nail <2 MOA on a Ruger Scout, and 0.75 MOA on a Savage 10TR, and 3 MOA on my XCR.
I also like the AR style mags and the left-side cocking handle on the XCR-M. I just didn't like it on the right side of the M305. I had a .223 rifle with a left-side cocking handle, and I liked not having to learn two different movements to work the action, change mags, etc. As an aside, I tried out a light barrel for the XCR, and it was not accurate either - about 5-6 MOA again.
All in all, I just got tired of M305s not functioning to my expectations. As long as I stick with the heavy barrel, the XCR just worked better for me, but maybe I just got unlucky with three M305s that were lemons all in a row. My buddy has one and it shoots reasonably well.