I own a Fireball V90. Its all MDF and Resin in the construction. There are a lot of good metal components on the machine like the shafts and screws. However, look at the price. $599 for the base kit, you add on the motors and electronics and you are at $1100. Add on your router and your mount and you are at $1300. I added more things on and got to about $1800 once I had the machine that I wanted. (I think overall with the computer and dust enclosure I spent $2500 on a Fireball V90.)
Looking at the Zenbot, and you have something that is much different from the Fireball. The Zenbot doesn't use traditional round shafts. It uses steel square tube at a 45 degree angle and uses rollerblade bearings to ride on that square steel. Instead of using an ACME threaded rod or ball screws, it uses a timing belt. This makes the machine much much faster. However the motors are smaller in power.
In order to price out a comparable machine between the Zenbot and the Fireball, you would need to spend upwards of $1200 (and then the Zenbot would still be smaller). Also looking at the elctronics for the Zenbot, they use a small voltage and small amperage power supply where the Fireball can get you up to 3amps per motor and 40 volts.
Basically the Fireball wins out in terms of power and using expensive accurate components. The only thing the Zenbot has going for itself is the HDPE construction, it definitely uses cheap components otherwise.
It might be worth investigating in an even cheaper third candidate which has a blend of Zenbot, and Fireball's features. Who knows, maybe that third candidate will offer upgrades to HDPE after listening to customer suggestions.
