The Reflex is a newer design, the Helix is really old, mid-eighties vintage.
The Helix is kind of funky looking, the Reflex is very sporty looking.
Running gear is similar but not exactly the same. Both are super reliable and durable. Somebody posted a Reflex odometer picture here with 80k miles! I own an '04 Reflex and can testify to its reliability. Just a great vehicle, gets 63 mpg commuting to work everyday 50 miles roundtrip.
I once saw a Helix on the interstate so they're also very roadworthy. Heard they are almost unkillable. Some guy ran a Reflex in the iron butt a few years ago and finished ahead of many larger bikes.