Dual brushless and a spool.

[RC-Monster Mike]

Allow me to share some input on this topic as well, as I have run a spool in the past. First, unless the tires are exactly the same diameter, the spool will not help the truck track in a straight line at all, especially on lower traction terrain. High power with a spool on loose dirt, for example, will wash the rear out everytime! The spool is a nice item for rock crawling, and is yougher than a regular diff, but unless it has been modified to accept 3mm screws (instead of the stock 1.5mm screws), it will eventually fail(the spool itself won't, but the 1.5mm screws will shear). If you want to track stright, you need a diff. Period.
Also, The RDLogics cvds won't work with the 1/8 diffs, unless you do some modifying.