The LPG should work out just slightly lower running cost than the derv. Its a case of factoring the purchase cost of teh two and then adding the LPG conversion
http://www.volvoforums.org.uk/showthread.php?t=135975
If you only do a low milage then the LPG might not be worth it. Short runs rule out any derv with DPF as if not run long enough to get hot they clog up.