Thermodynamically, it is BTU/lb. that matters. Suppliers will give you an ESTIMATE of their BTU/lb (typically 18600) but it often varies.
When Boeing or whoever sets out to make a record-breaking long-range flight, they first determine whether they are weight-limited or volume-limited. Based on this, they may select a high-density, lower BTU/lb. fuel like Jet A (for the volume-limited case), or a low-density, high BTU/lb fuel like the old JP-4 (for the weight-limited case). (I think high-latitude operators used Jet B, for lower slush point and higher volatility)
If volume-limited, they may also pre-cool the fuel to pack more pounds on board.