I have & use both - as Tatts says, RS is more popular and definitely more generous for the rewards points if you intend to share - but my MU speeds are far better.
It seems like RS is consistently under 100KB/sec - with occasional (1:5) spikes to faster, while with MU single link files (music or flix) I often get >1.2MB/sec.
But I don't use any of their 'managers'.
Get RS if you can only do one, just because it is the 'standard'; and I also remember that MU free was easier/faster.
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