You'll hear different opinions because every tank is different and has different needs. It's possible to have a successful tank with neither carbon or GFO but it probably isn't very common.
A tank that doesn't have many corals or not many that are different may do just fine with carbon in a bag. A reactor can be a good investment because a great many corals spend their time trying to kill their neighbors by secreting poisons into the water. Carbon in a bag is less effective in the same way that a DI cartridge sitting at the bottom of your RO bin wouldn't do much, but again if you have a light load it might be fine. I recommend using a high quality carbon like BRS Rox. There's a recall right now on Kent carbon because it releases heavy metals into tanks that it adsorbed during mining/processing. There's also some evidence that cheaper, dustier carbons can lead to head and lateral line erosion disease in some tangs. YMMV
Likewise some tanks do fine without doing anything to get rid of phosphates. But I recommend anyone setting up a new tank or having algae issues on an existing tank plan on having a phosphate plan in place, be it GFO, carbon dosing or both. Especially if you have a lot of fish, feed a lot or have an aquascape with areas that can't be cleaned. Like with carbon, sometimes you can get away with having it in a bag especially if you have a media tray in your sump, but since GFO is more expensive it makes sense to have a reactor to exhaust every molecular hole in the media before replacing it.
I'm personally a big fan of the BRS deluxe reactors because they install securely to a wall or stand, don't take up sump space, are easily configurable and it's very easy to swap media, especially if you have spare cartridges. You can regenerate GFO several times without ever taking it out of the cartridge if you have a spare reactor and you're into that sort of thing. The con of the BRS reactors is that some people have problems with them clogging or clumping. Some of those people report problems with particulates clogging the cartridge screens and slowing flow when the tank lacks mechanical filtration. Others just report 'mystery clumping' that is resolved by removing the cartridge and jostling it a bit. Again, YMMV but I personally am quite fond of these reactors, especially when driven by a return manifold with gate valves. I think the TLF reactors are a little wimpy and wouldn't feel comfortable leaving one unattended for a week.
