Here's where my head's at now. Subject to change.
1. If you find yourself looking at a bike that is halfway between a fitness bike and a MTB, pick one or the other. Half measures here are dumb.
2. If you get a MTB you should get one with an air fork and adjustable rebound damping and tubeless ready rims and 1x drivetrain and a dropper post, or at least frame cable routing for a dropper post. This category has long been thought of as the "$1000" bike but will now cost more, especially with 12-speed and a dropper. They're really good, though! Example, Trek X-Caliber/Roscoe family. If you only have Marlin money, that's ok, but you are missing out on some good stuff.
3. If you have a bike with a spring-only fork, top out the preload. The fork won't move unless you hit something.
4. If you get a bike with a lockout, leave it WFO and ignore it. It's solving a fake problem. If you have the cheap bike, see
#3 . If you have the "$1000" bike the rebound damping will keep the front from moving around much. I know it's hard to think of damping saving you energy but it's true, really! The only time I notice mine moving is when sprinting or climbing out of the saddle.
5. Don't worry about the weight of the suspension fork. It's about +3 pounds vs not. If you were really worried about weight, this is not the kind of bike you'd be shopping.
6. MTB's are more fun. Not-MTB's are
way faster.