Context : Upcoming spectroscopic galaxy surveys are extremely promising to help in addressing the major challenges of cosmology , in particular in understanding the nature of the dark universe .
The strength of these surveys , naturally described in spherical geometry , comes from their unprecedented depth and width , but an optimal extraction of their three-dimensional information is of utmost importance to best constrain the properties of the dark universe .

Aims : Although there is theoretical motivation and novel tools to explore these surveys using the 3D spherical Fourier-Bessel ( SFB ) power spectrum of galaxy number counts C _ { \ell } ( k,k ^ { \prime } ) , most survey optimisations and forecasts are based on the tomographic spherical harmonics power spectrum C ^ { ( ij ) } _ { \ell } .
The goal of this paper is to perform a new investigation of the information that can be extracted from these two analyses in the context of planned stage IV wide-field galaxy surveys .

Methods : We compared tomographic and 3D SFB techniques by comparing the forecast cosmological parameter constraints obtained from a Fisher analysis .
The comparison was made possible by careful and coherent treatment of non-linear scales in the two analyses , which makes this study the first to compare 3D SFB and tomographic constraints on an equal footing .
Nuisance parameters related to a scale- and redshift-dependent galaxy bias were also included in the computation of the 3D SFB and tomographic power spectra for the first time .

Results : Tomographic and 3D SFB methods can recover similar constraints in the absence of systematics .
This requires choosing an optimal number of redshift bins for the tomographic analysis , which we computed to be N = 26 for z _ { med } \simeq 0.4 , N = 30 for z _ { med } \simeq 1.0 , and N = 42 for z _ { med } \simeq 1.7 .
When marginalising over nuisance parameters related to the galaxy bias , the forecast 3D SFB constraints are less affected by this source of systematics than the tomographic constraints .
In addition , the rate of increase of the figure of merit as a function of median redshift is higher for the 3D SFB method than for the 2D tomographic method .

Conclusions : Constraints from the 3D SFB analysis are less sensitive to unavoidable systematics stemming from a redshift- and scale-dependent galaxy bias .
Even for surveys that are optimised with tomography in mind , a 3D SFB analysis is more powerful .
In addition , for survey optimisation , the figure of merit for the 3D SFB method increases more rapidly with redshift , especially at higher redshifts , suggesting that the 3D SFB method should be preferred for designing and analysing future wide-field spectroscopic surveys .
CosmicPy , the Python package developed for this paper , is freely available at https : //cosmicpy.github.io .