I would recommend diesel no matter how light the application is.
How old are your quotes? Are you getting quoted Tier III engines or Tier IV? If you are getting quoted Tier III engines you might want to check to make sure that you can still get them because all manufacturers are required to meet the Tier IV EPA emissions standard which is going to add $4000.00 to $6,000.00 to the average price of a diesel forklift truck. Some manufacturers may have enough components left to still offer the Tier III product.