An op-amp is more more resilient to both limiting and tolerating over-voltage spikes than a digital component. A quick swap of an op amp is much simpler than sourcing, programming and replacing a PICAXE, should either chip fail. Plus, if the price difference is important, the op-amp is cheaper (this reason is pretty minor, since the labour cost of replacing either chip is generally the largest part of the repair cost).IWP: Can you explain why the voltage follower is superior to a direct connection? Hoping to learn from your experience.
If you were using the "standard" fuel tank (i.e. designed to accommodate the sender) then it might be worthwhile to get the gauge as a reference for calibration. But if you are just using the sender as an "accessory" for your own tank, then you will need to calibrate the scale accordingly. Possibly, the standard meter would be an ammeter designed to be the upper "resistor" in a potential divider chain with the sender, (i.e. connected on the battery/supply side)..... any probs and I'll just order the gauge to go with it (it's not my money I'll be spending).