go with the HPI....here's my reasons.......
HPI parts can be found anywhere......The ball diffs in the TC3 are NOT a low maintenance item.........At last years HPI challenge,the Nitro stock class leaders were within 1 or 2 laps of the modified R40 class a main guys.....and that was a STOCK rtr...........(I should know,I won the nitro stock class)so there is no reason you can't race the RTR on a club level...The HPI is a stronger car........and will take more abuse.......
and the biggest reason?.......look the support team for HPI.....the web site......the hop ups....ALL the bodies that pop right on cause they are designed for the car...(not to mention the new HPI prepainted bodies)..
BTW,I have had plenty of associated products...and still own a nitro tc3...so I speak from experience....