If you are fancying proper crawling don't bother with the RTR. A slipper clutch and lots of plastic don't make a good crawler.
The ARTR has a few upgrades from the kit, but one part missing. The ARTR has the high clearance links for better ground clearance, the improved heavy duty diff lockers and the new improved straight rear axle. The kit still has the option of 4 wheel steering, although you can still fit the parts to the ARTR and 4 wheel steering isn't allowed for competition use. You also get in the ARTR a better body for more clearance than the kit one and Proline Hammer tyres with memory foam, which work much better than the kit Axial tyres.
And for electrics if you aren't running brushless the Tekin FX-R is the best choice for brushed, the drag brakes mean it just stops where it is when you let go of the throttle. If you fancy brushless then the Castle Creations Mamba Max and Sidewinder ESCs are they way to go, especially as
John Holmes will happily program them for crawling for you.