At the moment I'm torn between the Cataphractii kit and the Reavers.
The Reavers have the coolest chainaxes, and the haft looks just about long enough to work for a mounted warrior. Since I'd have to buy the whole kit, the marines can be turned into my 5-man plasma squad of Black Legion Chosen. A very expensive, but ideal option. Maybe for my birthday next month.
Second pick is the Cataphractii kit. The chainaxes look plenty good, but slightly less wicked. The price makes these a great choice. I think I'd extend the hafts just a touch.