With a B20, you replace the timing gear once, with
an alloy or steel gear, and never have to do so
again for the life of the motor. I'd rate the job
of replacing the timing gear to be about on par with
a timing belt on an OHC motor.

With a B20, when it overheats, you might cook the
rings but don't warp the head. I'd guess that you
are about as likely to ruin the rings of an OHC motor
when you overheat it, but you'll also warp the head.

The fuel economy is an issue, but there was a
thread recently about people with carbs getting 30mpg
on a B20. Mine certainly doesn't and my B30 gets
nowhere near that, but if other people can get this
then the B20 gets good enough economy.
chris