As far as the poll choices go, Percussion hardest, Guard not quite as hard, Brass least hard. This is based mostly on numbers, since a hornline makes up about 50% of most corps, guard around 25% and percussion about 23%, if each section ages out the same percentage of members each year, the percussion section will have the least spots available.
Percussionists trying out for one section are less likely to simply be moved to another section if they can't cut it (especially in regards to pit to battery and vice versa) whereas guard folk going for weapon line may only make the flag line, or a trumpet auditionee may be asked to play mellophone.
Add to that what others have already said about the 'cool' factor of being in a snare line, and percussion is definitely the most competitive section. This is probably even more apparent in some of the smaller corps that struggle to recruit a full hornline, but have relatively no problem finding folks for the battery.