We will need to travel between Alice Springs to Gladstone next August without flying.

Trains could be interesting but this seems to involve an obligatory overnight bus ride from Alice Springs to Mt Isa.

Renting a car (or is a 4x4 necessary) is an option. In your opinion, which route would be most time efficient and enjoyable:
Alice Springs due east to Longreach, or
Alice Springs, via Tennent Creek and Mt Isa, to Longreach?

Thanks in advance for your help!

To take the 'direct' route to Longreach, you would need to take the Plenty and Donahue Highways. We have been on part of the Plenty Highway, which is sealed for the first portion, but then turns unsealed and becomes quite rough. We did this part in a 2WD van, and it was OK, but bone-jarring. I'm told that the Donahue Highway is worse still and not at all recommended for 2WD vehicles. The last part would be on the Kennedy Road, which is also rough in places.

If the goal is to travel outback roads, this would be one way to do it, but most of the way is extremely flat and uninteresting. There would be better choices for more scenic terrain. I also doubt if you will find a company that will hire you a car to take this route.

If the goal is to get from Alice to Longreach, then it will be faster to take the well-sealed Stuart Highway to Three Ways, then the Barkly Highway to Cloncurry, and the Landsborough Highway to Longreach. That last one is sealed but not always in the best of shape. It is generally not a very interesting trip, with many long, straight, and flat stretches, and you will have to periodically contend with giant 'road train' trucks. Still, it is a far better way to go than the 'direct' route.

I can't speak to the train/bus idea, as I've never done it.

Thank you very much ALF that is exactly what I needed to know. I'll take the route to Three Ways and then get to see the devil's marbles and Gemtree (between the road trains)!