Just wondering which route would be more memorable for this journey and perhaps a recommended coffee stop?
If you have the time can highly recommend driving through The Catlins.
Some stunning bays/vistas. Just a really nice (and quiet) place
The Catlins is a serious detour from QT so unless you have a few days to do the journey, don't go this way.
The fastest way is via Cromwell/Roxburgh/Milton. I like to go via Cromwell/Omakau/Ranfurly and there's some nice little spots to stop off on the way if you've got time, ie St Bathans & Naseby. Try Oliver's in Clyde for lunch/coffee
Here are some of the attraction on the way.
Mclean Falls, Cathedral cave (must be low Tide),Florence Lookout, Matai falls, Horseshoe Falls, Purakaunui falls(one of the best in NZ), Nugget point lighthouse, Lost Gypsy Gallery
Since you only have the day to get Queenstown to Dunedin, go one of the two routes QTN has offered. The road along the Clutha River (the route Cromwell/Roxburgh/Milton) is really pretty and I think underrated. If you want to continue with the river scenery, turn right onto Rongahere Rd, just before Beaumont towards Balclutha. It will add about 20 minutes to the entire journey. Since you are approaching from the south, head to Tunnel Beach and have a look around.
Only go via the Catlins if you can spend a night or two there. We spent a night at Curio Bay and the next at Nugget Point and spent the entire day seeing the attractions in between. You really can't do it justice as a drive by because the attractions are down side roads.
As we only have the day to get to Dunedin will follow the Clutha route but appreciate all advice. We will be moving to Mt Cook after Dunedin so maybe we will see some of the other picturesque route then.
Like the others have said the most scenic drive is via the Catlins but to make it worth while you need a few nights. Last time we did the trip this way we took a week.
The most scenic route is to fly, either small fixed wing or chopper and makes the best use of scant time.