What a Mondeo ST you mean?
theres a serious cost difference between the ST220 and the ST TDCI
theres also the fact the ST TDCI is an oil burner and only develops 155bhp.
If you just want a car thats not rubbish, the ST TDCI will do you fine
but the ST220s are now in the £430 a year Tax bracket. ST TDCI is £175 a year. ST220 gets about 20mpg in the real world. ST TDC is quoted at 47mpg but will probably get 40 real world mpg. But thats still double the fuel efficiency of the ST220.
Then theres the fact the ST TDCI is lower insurnce (grp 13 vs 16 for the ST220) so will be cheaper to insure as well.