They are not the same. With the DI you can convert your high impedance unbalanced guitar output into a mic level (and low impedance, and balanced) signal. This signal must be amplified to to be useful by means of a mic pre-amp, so you will need the pre-amp as well. This is a good option for live performance applications.
When using the pre-amp in instrument input mode, you are configuring your preamp so it can properly receive and amplify the guitar (or whatever) output. With this kind of pre-amp you will not need any other device to record a high impedance signal. This can be a more suitable option for recording situations.
It depends on what you want to do.
They are not the same. With the DI you can convert your high impedance unbalanced guitar output into a mic level (and low impedance, and balanced) signal. This signal must be amplified to to be useful by means of a mic pre-amp, so you will need the pre-amp as well. This is a good option for live performance applications.
When using the pre-amp in instrument input mode, you are configuring your preamp so it can properly receive and amplify the guitar (or whatever) output. With this kind of pre-amp you will not need any other device to record a high impedance signal. This can be a more suitable option for recording situations.
Hope this can help you.
Miguel