I think prices for performance are relitively the same like on new egg the other day i saw a E6400 and a x2 5200+ were both around 200 the E6400 only $17 dollas more if i remember correctly ... with performance there about on par some better than others on benchmarks.. but the thing that would get me to buy the E6400 is its overclockablity. u can get way more performance outa the C2D than u can with the x2 and FX ..
So for overclocker's i'd say deffinately C2D
for non-overclockers either, either ... AMD is cheaper atm for performance i think.
If u think u might overclock in future ... go with C2D
thats my 2 cents