The choice would purely come down to performance and cost. It's almost a guarantee that NPM will perform better on newer, faster hardware. If you already have the hardware / budget- go for it.
Otherwise, if performance is presently good, stick with your current hardware. We're not like Apple iOS upgrades in terms of HW requirements between versions.
If you do decide to move to a new box, here is the migration guide: http://www.solarwinds.com/documentation/Orion/docs/OrionServerMigration.pdf
Clik here to view.
