I guess for me it would depend on application and the skill of a person who could narrow them.
If you could get a real good price for your used pair, a new set would be the ideal way to go, safe and you have accountability to a manufacturer if there is an issue.
However if you know of a master fabrication shop that could narrow and tig weld them back togeather that is certianly an option. But they better have a hell of a good reputation for such things.
Remember you trust your life and the lives of others on the street to those wheels. should you have them narrowed and the welds fail, you could potentially wreck your car, motor, hurt yourself or someone else.
I would say if your car is worth 20K plus dont mess with narrowing.
Just my opinion