Neither is better, though for a given application one might be. A good Aluminum seat is fine. it's the economy type of seat where it's basially you and some 18Ga Aluminum sheet held in with some pop rivets and a harness that are a problem.
When it comes right down to it, if you are an odd shape and don't fit well into an FIA seat, get a good ally one from one of the known manufacturers (buttler, Lajoie, ultra-shield). if you fit an FIA well, it's probably a push as to price and safety AT THE CONTAINMENT LEVEL, so get what you prefer. If you can't afford a good alluminum containment seat, get one of the newer budget 8855-1999 FIA seats with wings, like a OMP RS-P.T.2 or Momo Daytona, both under $800. Good aluminum containment seats get comparatively pricey quickly - and there's a lot of research to support their effectiveness. while I'd never suggest an SFI cert is actually useful on it's own, a 39.1 compliant seat should be a very safe place to sit. don't be turned off by a comparable seat without the cert. do not buy any seat that is NOT obviously comparable to an SFI 39.1 seat, nor an FIA style that is not 8855-1999 or the newer standard 8862-2009, which is the best type of seat you can buy IMHO.
Also, consider what Greg said, as well as your mounting options, the amount of room you have to the sides of the seat, etc... in your application. Something as simple as a transmission tunnel being very close to your leg as seated could make the decision for you (FIA tend to have very thick side flanges to create strength from the composite sheet while fabricated seats can strengthen where and as needed and can be much less wide in certain less critical areas).