The thing is you can either play safe and opt for a subject that a majority of other candidates may also opt and hence be relative secure from examiner's dropping the sword on that subject or you can opt for a subject which has a higher failure rate but also higher avergae pass marks. For example, as I.R is incredibly popular(60-75% of canddiates go for it as 200 mark paper) setting yourself apart from others may be very difficult, in which case you expect a range of 80-110 marks depending on your prep. However, as a benefit failure rate for I.R is usually less that 25% which means it will not impedee your success scenario. Unique options give you higher highs as well as lower lows. Two out of the top three in 2018 had Pol. Sci. as their optional and scored 140+ but a larger percentage fail in this paper as well.