from my limited experience both are nice engines and provide a reasonable amount of grunt, however it might be wise to do the following (I'm not going to do it for you so I can't be blamed of getting it wrong ;))
1) find out the exact make and model of the engine and then try asking uncle google for a full set of tech specs for it. Then compare this against the Tdi, the 2.5NA you already have and so on.
2) find out *exactly* what is needed to fit the engine into the motor, from what I recall they will both fit they will require a conversion kit which costs money (I was quoted around £300 for the isuzu kit) which again takes some of the shine out of the engine...
