  Angular differential imaging (ADI) and spectral differential imaging (SDI)
are commonly used for direct detection and characterisation of young, Jovian
exoplanets in datasets obtained with the SPHERE/IFS instrument. We compare the
performance of ADI, SDI, and three combinations of ADI and SDI to find which
technique achieves the highest signal-to-noise ratio (S/N), and we analyse
their performance as functions of integration time, field rotation, and
wavelength range. We analyse SPHERE/IFS observations of three known exoplanets,
namely Beta Pictoris b, 51 Eridani b, and HR 8799 e, with five differential
imaging techniques. We split the datasets into subsets to vary each parameter
before the data are processed with each technique. The differential imaging
techniques are applied using principal component analysis (PCA). The tests show
that a combination of SDI and ADI consistently achieves better results than ADI
alone, and using SDI and ADI simultaneously (combined differential imaging;
CODI) achieved the best results. The integration time test shows that targets
with a separation larger than 0.24 arcsec observed with an integration time of
more than 10$^3$s were photon-noise limited. Field rotation shows a strong
correlation with S/N for field rotations up to 1 full width at half maximum
(FWHM), after which no significant increase in S/N with field rotation is
observed. Wavelength range variation shows a general increase in S/N for
broader wavelength ranges, but no clear correlation is seen. Spectral
information is essential to boost S/N compared to regular ADI. Our results
suggest that CODI should be the preferred processing technique to search for
new exoplanets with SPHERE/IFS. To optimise direct-imaging observations, the
field rotation should exceed 1 FWHM to detect exoplanets at small separations.
