I have both a Circuit and a Rustler. Of the two the Circuit is more stock. The Circuit parts are very flexible which depending how you look at it is a good or bad thing. The Rustler most of the stock parts can be bought cheap but better to go ahead and upgrade like RPM when they break rather than replacing with stock parts. Broke both shock towers, rear and front a-arms, front and rear carriers, caster blocks, bent kingpins, had screws constantly back out, front bulkhead, shock caps pop off, cracked chassis, broke top shaft roll pins, stock driveshafts maybe more don't want to keep thinking about. The Rustler also uses different offset wheels front and back you can't just buy all four of the same wheels and switch them around. The Circuit IMO is tougher as far as stock parts go but has it's weaknesses. The driveshafts are worse than Traxxas shafts (IMO), servo saver sucks, shoulder screws for steering knuckles keep coming out, plastic diff, the metal gear upgrade can't buy each individual parts separately (diff, idler, top shaft gear) those are just off the top of my head. The appeal of the Rustler was the parts and aftermarket support you can dump a lot of money into it whether you want to go ahead and upgrade when something breaks. If it was brushed I would of probably broken less parts.
The Circuit I have is actually from when they used to be called Electrix. Over time things were upgraded but still using most stock parts. Not for a lack of upgrade choices just that stock parts aside from the servo saver, driveshafts and shoulder screws for the steering knuckles aren't that bad for a basher. In all fairness to the Rustler after the first break I usually just upgraded the parts maybe when the parts broke could have been a one time thing (i.e. bad landing, crashing into something) and another stock replacement would have been fine don't really know. My HPI Blitz and E-Firestorm since they are basically the same have been the most reliable as far as stock parts go.
Best fix I have found so far to those stupid shoulder screws popping out is the hot racing front spindles.
Having owned both a Traxxas Stampede 2wd XL5 and a brushed Ecx Ruckus 2wd for the last 4 years I can attest that in general the Ecx is a bit tougher. And I agree on the axles, I just used stock Traxxas Slash 4x4 rear axles ?.
The shocks for both vehicles are rather crappy and will pop the caps.
I kinda miss my Firestorm ?