As far as I can remember, the main mechanical differences in the game are that turretless AFV's are less likely to perform op-fire if flanked compared to turreted vehicles. They are also generally (but not always) cheaper than their turreted cousins. If any of that is incorrect feel free to slap me around a bit, Don.
Incidentally, in some cases it would also be rather more 'historical' to field assault guns than tanks to support your non-motorized infantry. This is especially true for normal German Grenadiers and Russian line infantry past 1942 (SU-76's especially).