hmm, well they both cost around the $1000 mark I suppose. I did coilovers first as I hated the 4x4 spec plus wanted that extra reponse with handling. I would say the extra power with nistune would be more usable with coils installed, at least around corners. Car definitely feels safer and more responsive with them.
Probably get my nistune done in the new year so will let you know how I go.