Well, I'd like to chime in here and yell at some folk
I've been a MTB designer for 15 years or so now, just for context - not dick measuring.
My only bike is a 27.5" hardtail, not because it's more efficient, but because I like it. It's not faster than a comparable FS either up or down hill, except maybe on smooth trails without big features (jumps, drops, etc).
If I were to advise someone joining the sport, I'd say get a 29er hardtail with good geometry. Geometry is king. Upgrade tyres if desired, but don't just target light weight. After that I'd probably upgrade the forks. Get a reliable dropper post.
Hardtails are cheaper, like for like, and will teach good riding habits.
However, if anyone wants to go fast up or down hill, the answer is FS and money. Like I say, a hardtail is only faster on smooth trails, assuming both are well designed.
My next bike will likely be a 120 or 140mm 29er FS, in at least a year. I love my hardtail, but my ankles hurt and it's annoying to have to drag around a DH rear tyre with a cushcore insert, and occasionally still pinch flat.
Oh, and avoid plus size tyres, unless you want a laugh.