My 10p... if you use a chassis then any armour you bolt to it can easily be repaired/replaced where as if its a monocoque then any damage to the structure is much harder to repair.
On the flip-side, monocoques are harder to rip apart with say spinners as they can be built with stronger joins or welds. The panels on a chassis are collectively strong but if one it removed then the others are much more vulnerable.

Bigger brother Vs Hypnodisc is a great example of a chassis and panel set up, as for monocoques... I cant think of one... Wheely Big Cheese perhaps?