Rega Elex-R for me, no contest. I haven't heard the M3si with the LS50, but I've heard it with M6si; Rega is better in terms of subtlety and refinement of highs, as well as more controlled bass. M6si was just too bright, and I got listening fatigue in less than an hour.
If you don't need the extra wattage, you can also consider the Brio which has almost the same sound sig as the Elex R. Just a little bit less refinement, and a little bit less dynamic, but you wouldn't know the difference if you don't compare it side by side.