your not gona get much speed from any defender, but the TDI engine is 111bhp and 195 lb ft compared to something like 80bhp and 150 lb ft for the TD.
Also the tdi has a much stronger block so will cope with a bit of messing eg. larger intercooler, fuel tweak etc.
If you can afford it get a TDI, but bear in mind it is easier and less work to buy a TD with a good chassis and bulkhead then stick an ex disco tdi in there, rather than get an original defender which is messy, needs welding etc.