I’ve worn bell, garneau and Giro in the past and tried Poch, 661 and lazer. They all fit differently. Some don’t fit my head at all. Best to try them all on and see what feels best. I find that Specialized fits the majority of people best.
I would also suggest something with the MIPS technology. It’s designed to help lessen the impact and help with the severity of concussions. Does it work? Well let’s hope one never has to find out, but if it helps a little it’s worth the extra 10-20 buck imo.
For MTB I would definitely suggest something that does extend down the back a bit. Keeps the helmet in place in the event of a crash.
One other small thing. If you ride at night, make sure you have holes in the right area for your head lamp if you use one.