In my view its all about how much your value your QFF points at. I value mine at 3c each so the calculation is easy:
4c / litre discount, 60l fill = $2.40
2p / litre, 60l @ 3c/point = 60 x 2 = 120 x 0.03 = $3.60
If you value your points at say 2c each then the result is neutral, 60l x 2p = 120p x 0.02 = $2.40 In this case I'd still take the points as I struggle to accumulate enough points yearly for all the things I want to do and having a float of at least 100K points helps a lot.
Financially, if you value your points at less than 2c ea you'd be better off taking the discount.
Incidently, the above guidelines also work if you are wondering, at the point of sale, if you should cop a CC surcharge. If the merchant wants more than 2-3% I can't go for it, as my points are only worth 3c each to me. My power bills cost me 0.65% so is easily "affordable" and worth the points gained (assuming I gain 1QFF per $ spent).