In general there isn’t a huge difference between the two and in fact any device that is suitable for propane should be suitable to use with butane as well (and vice versa).
If your appliance is able to use both the only change that will be needed when converting from one gas to the other is swapping the regulator to ensure correct flow rates for each gas. In some cases some appliance manufacturers may make it hard, or even impossible, to change this regulator. If you are unsure it would be best to check your appliance handbook.
There are essentially two main differences between the actual gases; price and boiling point.
Price wise the simple answer is that per KW/h butane is (in general) cheaper than propane as more butane can be stored in a cylinder of a certain size than propane due to propane exerting a higher pressure at similar temperatures than butane. This means that, as you may have noticed, a tank holding 4.5kg of butane will only hold 3.9kg of propane whilst the price per cylinder is usually about the same.
In a cylinder of either propane or butane both gases are stored under pressure and the majority is in liquid form. When a valve is opened any gas in the cylinder is released and, as the pressure drops, the liquid begins to boil releasing more gas. What this means is that if the temperature of the gas is below or around the boiling point of the gas it will not flow as readily (or possibly, at all) and will be unusable. For propane this is generally not an issue except in very extreme cases as it has a boiling point of -43.6 degrees fahrenheit (-42.6 centigrade), however, for butane with a boiling point of 30.2 degrees fahrenheit (-1 centigrade) this can be a common occurrence even in a relatively mild winter.
Our general view is that if you are 100% sure you will not be dealing with freezing temperatures then butane is a good choice, however, if you have even the slightest doubt about this then we would recommend the use of propane, especially for outdoor tanks.