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As Umar said the real response would stand somewhere in between a pin and fixed base, similarly extent of column fixity at base depends upon the rotational flexibility of foundation that depends on foundation stiffness and soil stiffness both. In general it can be said that for rigid raft foundations,foundation supported on stiff piles or basement walls this rotational stiffness is high and one can confidently assume fixity of column at base. But in conditions of individual footing pads on deformable soil,foundation could have considerable rotational flexibility and consequently assumption of fixed column base could show a considerable variance with respect to distribution of column moments on bottom storey i.e in such case column moments could be concentrated at top end of bottom most column rather than at base. Therefore, It is recommended to model the rotational stiffness of foundation rather than assuming fixity at base in cases of individual column pads on deformable soil to represent quite realistic partial fixity available at column base.This methodology can be seen in "Seismic design of reinforced concrete and masonary building by M.J.N Priestley" (page 466).

W.salaam. Since we normally don't consider soil structure interaction in structural analysis, therefore I am quite certain that the statement quoted by you about choice of pin or fixed based on soil type is flawed. Anyhow, for concrete columns unless special detailing is done at coulmn joints, they would always transfer moment that should be accounted in the design. Using fixed based would likely result in conservative design for structural members than using pinned base. The real response would stand somewhere inbetween a pin and fixed base. Thanks.