I have both 300R and SR15. I've been using both extensively over the past 5 months. I think the SR15 beats the 300R in detail, resolution, bass and mids. The 300R has a more airy sound to it and perhaps a little wider soundstage. The SR15 is more mid-focused and more organic sounding, while 300R is more neutral in my opinion.
The 300R, despite using an older android, is very fast even with current updated apps. The M11 from FIIO is the only android-based dap I've tried that's faster than the 300R, but the M11 doesn't do bass quite well in my opinion -- a little loose, uncontrolled and missing resolution. These opinions were based off of using all three with CFA Solaris, qdc Anole VX, CA Fibae 7, Meze Rai Penta, Etymotics ER2XR and Moondrop Kanas Pro.
At the end of the day, I prefer the SR15 over the other two, but it also does cost more (present day value). I am selling my 300R now, and that's mostly because I prefer the portability and size of the SR15, plus the slightly more engaging sound.