I have run both. The Zen is simpler and if you drive it properly, faster. If you throw cars around like a TC, overdriving it and hauling on the brakes, then it won't make much difference either way.
The speedo is a key part of going fast enough, so money invested in a good one (HW, GForce, etc.) will pay back in good throttle feel, speed and punch as well as a good price if you want to sell it on.
The Turnigy and Sky Toro get good feedback so they might be worth a look, but I've not used either. HTH
