Which 360 Block to use: LA or Magnum?

Magnum provides roller cam, plus metric lower tension rings

Plus the stock rotating assembly is lighter in the Magnum than in the LA.
You can Bolt the LA water pump on the front of the Magnum get rid of that reverse rotating crap that way. Buy the little adapter piece that goes on the front of the camshaft and you can run a mechanical fuel pump then you put old style valve covers on it and nobody knows it's a magnum.