A few things to think about:
What seats are you running?
What cell?
Stock or flat dash?
Center console?
Harness or belts?
Cage tie-ins?
As far as which route to take, custom all the way. Production cages are built to suit a broad spectrum of configurations but sooner or later (if not immediately), you'll find yourself wanting to redesign a few things to suit your needs. Cages that fit a typical Corbeau SS may not work with Ultras. One designed to contour the stock dash may look hideous if you ever wanted a flat dash. A production cage might work well with a RCI 2161, but is that the only cell you would ever run?
A good fab shop that builds to individual specs will ask certain questions to ensure its exactly what you want. It'll take longer, yes, but at least you won't have to cut it all up two weeks later and spend a few hundred dollars making it perfect.