Hmmm tricky one.
A year or so ago I would have said Selmer as personally I thought they were nicer. However, I tried a new Buffet not that long ago and it was absolutely gorgeous!
I don't think you can go far wrong with either.
Personally I purchased the Yamaha to low C. Whilst not as nice as either the Buffet or Selmer it had the advantage that I could afford it! I didn't expect to be doing anywhere near as much bass playing as I have done (I'm an Eb player by preference if doubling) but the Yamaha has been up to all the challenges I've put in it's way. It also has better key spacing for my little hands including a low D key for the thumb.
I've upgraded the mouthpiece to a Weinburg (

) which had an amazing impact on the projection and helps a lot with the nasty tricky notes on any bass (throat Bb, bottom B, clarion F# and all the notes just above the break).