The process of selecting valves for a plant handling LPG (both liquid and gas). Normally for larger sized valves (4" and up), the double eccentric, firesafe BF valve is a good choice, especially due to dimensions/weight and price-wise. Customer is worried that the seal on the BF is more exposed to impurities in the cargo than a BV would be. I have only experienced problems with seals due to poor cleaning of pipeline before start-up (welding threads/beads, particles).
If you check references from some of the well-reputed valve producers with many years experience from the fluid, you will find that your opinion is correct.
In my own opinion seals in a ball-valve would have limitations in construction to accept a very high degree of impurities where special consruction BFL-valves for the purpose probably will have better tolerance for impurities(Triple eccentric BFL, special sealings and other special constructions)
Cost, cost over lifetime and practical solutions will however (for both types) as for all valves be depending on the detailed process conditions and requirements at site, and availability and commercial price locally for the particular valve.  

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