I'm on the same path as you. Roadie for a long time and just now getting into mountain biking. Like most above, my bike shop advised to going flat until I have more experience. I figure about a year or so and I'll make the switch to clips. Shimano has a great all metal pedal that my bike shop had for only $60. Comes with 2 sizes of washers to help adjust the stud length.
https://bike.shimano.com/en-EU/produ.../PD-GR500.html