at stock speeds the 2.4c will mop the floor with a 2.4b.
when overclocked to 3+ghz there isnt much if any difference tho.
the 2.4c's HT may help in some apps, but it also hurts in some others.
id suggest using your 2.4b for awhile, if youre unhappy with its performance ~ overclock it! (or get a C chip)
HTH