They are 2 different methods. R128 uses LUFS model for normalization which also takes into account perceptual loudness vs Repalygain which looks at a target db and measures from there.
From my experience, you can have either tags for FLAC and symfonium will pick them up even when trasncoding because all it looks for is the tag there, yes = apply value no = continue, just dont have both tags there like I did for some files. 
Here is that issue I had with double tags if you want to do some more reading 