They do completely different things. One is good because it's a cheap, fast, comparatively durable troop with twin-linked almost rending guns. The other is an incredibly annoying denial unit that is more mobile (skilled rider), can provide solid anti-tank projection (Heat Lances or Blasters), plus a strange and good "shooting" attack that removes casualties from the rear of a unit. I would also argue that Reaver Jetbikes are "tougher" because they have the Power From Pain special rule. Reavers can also be better in close combat because of Combat Drugs plus an Arena Champion with a Power Lance or an Agonizer. Windrider Jetbikes, by contrast, have access to Warlocks.
Personally, I don't think it's good comparison because Eldar Jetbikes are "better" in the sense that they are always scoring but are otherwise one dimensional.