I have only done Integrations from other databases, never tried a text file, outside of testing the Fixed Asset integration with 8.0.
It seems pretty straightforward though. The ACCESS integration would probably give you more power because I imagine you could have more power over querying source data, then you would say in a text file?
As far as performance, if you could eliminate the network overhead, i think you would be doing yourself a favor. If you could export to text file, then store the text file in the same location you are running IM that would be good. Or, take the .mdb access file and move it to the server...
If you have existing Integrations, I would look through them and try to identify how they are set up.