Inogens are good. They are pricy.
The G3 produces a maximum of 5 liters pulse. Note that pulse is effectively less oxygen than a constant flow concentrator, so if you use, say, 3 liters on your constant flow, be prepared to set the G3 on 4 on a pulse machine. (Last I checked, all of the portable machines are pulse. The large units, plug in the wall, are constant flow.)
Inogen is the only company that we know of that has a machine that will do 6 liters pulse, namely, the G2 model. That maximum setting is just enough for my wife if she is talking, but otherwise sitting fairly still. And it allows us the freedom to travel.
If you go with Inogen, buy the lifetime warranty. It's worth it.
Check to see if your insurance will pay for all or part of it. Ours paid for exactly none of it, but it was worth the investment nonetheless so that we can travel.
Also invest in an extra battery; an external battery charger (you wouldn't think you'd need it until suddenly, unexpectedly you do); and a 12 volt charger.
๐Lynn