Try to find out what plate voltage and what current is applied to the output tube. Then you can calculate the plate dissipation in Watts. Then find out what are the manufacturer's suggested max plate voltage, current, and dissipation. If the plate voltage, plate current, and the product of the two (Watts) are not more than 60 to 70% of the respective maxima, then the lifespan of the tube will be at least not reduced by the circuit demands. Within those limitations, you might get an idea of which of the two types you mention will last longer by looking for the manufacturer's estimate of lifespan. Just keep in mind that the numbers given are "on average" and may or may not apply in a given instance.