As said before, horses for courses. Depends where you ride, how often, and for how long.
Had a stiff alum HT for over 10 ten years, and rode it on everything, XC, downhill, the lot, as everyone did back in the day when dualies were too expensive.
Have now owned 4 different dualies in the past 7 years, 4 to 6" travel bikes. The performance now is amazing, you can do everything on a 5" travel bike.
If you are strong, skilled and riding for a short time, the HT is generally faster for many reasons, but soon beats you up. A good modern dualie is almost as fast, but doesn't beat you up.
My lower back loves me now, but hated me on the HT after 1-1.5hrs on the bike. You can use your legs to suspend your arse over bumps on the HT, but that soon tires your legs. Meeting able to sit down and pedal through smaller bumps on a good dualie saves your legs and back, and you can ride for longer.
Recently rode a mates carbon sick fast specie 29 HT after coming off my blur XC 27.5 bike. No comparison, the 29er HT felt F1 fast when pushing 100%, but when just riding along no real difference in speed. The HT felt harsher and more work to hold a line with, so was glad to get back on the blur.
If getting a HT then obviously get a 29er for speed, but if a dualie I honestly think it is either 27.5 or 29 depending upon your height and types of trails. If a lot of firetrails then 29, if more single track then either is fine.