5 Qualitative considerations

0. IFS provides 2D spectroscopic information
1. IFS always works in cumulative way. Previous comparisons have been done when observing galaxies at a given magnitude (+/- 0.5 mag). IFS will give, in addition, spectra of all the brighter objects in the field.
2. MOS has a better spatial resolution (in this comparison)
3. MOS can reject bright stars/galaxies in the FOV, minimizing scattered light (from this potential source) inside the spectrograph
4. IFS may have better spectrophotometric stability due to absence of moving parts. In addition, it is insensitive to pointing/tracking errors.
5. IFS data should be easier to background subtract. This will be especially relevant in crowded fields or with and imperfect PSF
6. MOS produces more diffracted light (the wavefront is cut in two directions). This and the packing fraction of the MMA (or equivalent device) may influence $\epsilon_{mos}$ and therefore the relative sensitivity (see equation 1).

For more information, please send mail to Santiago Arribas.