do yourself a favor
For a 10 year old, you probably need a durable car that is relatively easy to work on. Do yourself a favor and get your son a used xray t2 or t2'007. You can buy a good one for about $150. The xrays are durable, well made and easy to work on. The parts aren't cheap (cheaper than corally, more than tc4,) but it's not easy to break an xray car.
As for the cars you mentioned, the tc4 is a shaft drive drive that is not very smooth to drive (because of the shaft.) Also, if your son will be racing, the tc4 really isn't very competitive because it has a completely outdated design. The RDX is a good car, but it eats front arms, and parts are very expensive because you buy them in packs, versus individually. For a 10 year old, a couple of broken parts will break the bank. BTW, the arms are 4 for $29 + shipping. With xray, you can buy the arms individually ($6-$7), and they are very hard to break.
Unless you are can get the tc4 for $80, and that's your budget, get the xray. I've owned the cars you are looking at and I was very disappointed with the tc4. The corally was simply a money pit, and a pain to work on. I currently own an xray t2 (original) and a 008 and frankly for a 10 year old, there isn't that much of a difference between the 2. One more tip: if you buy the xray, get one with a bunch of spare parts. The 008, 009, and t3 don't share many parts with the t2 and t2 007, so you can get a great deal on a used one with a bunch of spares and hop ups.
Good luck.