Maybe this is six of one, half a dozen of the other. I think that lawyers per billion lets someone more quickly compare their metric to another company’s metric or a benchmark survey. “If the figure is 5 per billion and we are 2 billion, we should have 10 lawyers (2 times 5).”
However, revenue per lawyer is more grounded in reality and seems more tangible. “If the metric is 200 million per lawyer, and we are at 250 million per lawyer, we are a bit ahead of the benchmark group.”
You pays your £€¥$ and you takes your choice.