The feel of the vehicle is very different. Buggies often have a centre differential whereas many trucks replace this with a 2-speed box and/or a slipper clutch. This is not a universal rule, however. Truggies can take either form, however they usually go for the centre diff approach.
A truck has a higher CoG and has much more ground clearance and can usually take bigger jumps without causing damage. A buggy runs much more like its on rails and really sticks in turns thanks to the centre diff, where the back end of a truck would be more likely to let go. Wheelies are obviously impossible in a buggy
How about
this? Very reliable .21 block with the 2-speed box. Completely RTR, so comes with radio/servos/engine etc. Very long stroke shocks so they are great for landing large jumps. The Savage series have been around for ages and are extremely popular.
Also that kit has the hardened diff gears kit - very useful when you consider how difficult it is to get at the diffs on a Savage (one of the main disadvantages of the twin vertical plate chassis design).
Edit: Quick pic of my TNX