it is the pulling power of an engine, the difference will ideally be known when one closely observe the difference in initial pickup. final pick up may be the same for both engines as the difference will not widely be known. Normally firing of the diesel will be slow when compared to petrol. therefore higher horse power engine gives you more power when initial pickup is demanded which suits city driving , where frequent breaking, start and stop is more. an engine with 110 ps with AMT is a good bet for city driving.
Both cars are awesome and it is indeed hard to pick one of them. Duster is a bit more expensive than Ecosport. However, Ecosport has a lot on offer as for the price of a mid variant (85 PS) of Duster, you have the choice to go for Top end Ford Ecosport. It generates better power and also is great to drive than the 85 PS version of Duster. If you are going for the 110 PS Duster is going to cost you more. Hence, Ecosport is a better choice here. It also has been introduced with a new infotainment screen and other features in the platinum edition which would be the best buy for you as of now. Click here to check out the details of the platinum edition and here to check out the comparison.
PS is the German unit of power, 85 PS means that this car generates 85 PS of power and RXE is the name of the variant. It has got a 110 PS variant also which means that the engine is tuned to generate 110 PS of power.