Many, many years ago, Jaguar ran a 3.4 litre in convoy with a 2.4 litre (obviously dictated by the performance of the 2.4 litre).
The 3.4 litre returned better fuel consumption (no doubt due to the longer gearing than the 2.4 litre and lower overall revs).

This could mean that, for the same performance, a larger capacity (and suitably geared) B9 could better a B7 for consumption.

Of course this would depend on the relative degree of sophistication of the emission control.

Lifetime of the smaller capacity engine might be lower than the larger, less-stressed engine (or, expressed differently, the larger capacity engine might be less troublesome in later life and therefore last longer).

Just saying. Actual experience would depend on route performance duty.

A good biggun will beat a good littlun.