speed differences between the 350 and 500 are minimal, they are both slow.
one is marginally slower than the other.
i used to run both a iron barrel 350 and a 500 as a daily transport for years.
doing a 40 miles round trip every day.
with regular maintenance they are completely reliable.
at the time the only issue i had was the spare parts quality, parts were stupidly cheap ( 12 euro for a 350 piston? )
but quality of indian aftermarket parts varied from completely useless, to pretty good and it was hard to judge what was the good stuff.
nowadays with proper importers , and factory parts available this should not be a issue any more.
i now have a daily 50 mile round trip, all of it highway , and a guzzi as daily transport.
but with a job closer to home i would probably still be running a enfield.