Hi Wijnand,
Please find the difference for each of the fields below:
EffectiveFinalUrl: This will return the actual effective Final URL for each impression. ExpandedFinalUrlString: This will return the Final URL with ValueTrack parameters replaced with their final rendered values.
The results from the EffectiveFinalUrl and UnexapandedFinalUrl may or may not match. Let me know if you have any other questions.
Regards,
Bharani, AdWords API Team