Ampex used both metal and ferrite ("glass" in the day) heads. Their glass heads were seen as a tremendous benefit for radio stations, because they were hard enough not to need lapping, except many moons down the pike. They didn't sound spectacular, but they were dead reliable. If a ferrite head cracks, as can happen if they are over-heated by an oscillation in the audio circuit or something, or if someone bangs a flange into them, they can literally scrape the oxide off a tape or cut it in two.
Ampex metal heads sounded great, but were quite soft and needed lapping regularly. The newer Flux Magnetics heads sound better yet in my opinion, and I would recommend them.
Bear in mind that your playback head only matters to the extent it allows you to listen to your recordings, unless you intend to master from that machine as well. It pays to make sure your record head is in impeccable shape first, because that's what is making the recording.
Half-inch gives better bass response and less noise than quarter-inch, all else being equal, but may have slightly less stable azimuth. That's about the only trade-off, and I think it's a worthwhile one.
Good luck.
