There are a few things to consider:
The first is more software/hardware - the NRF24L01 uses a proper protocol, is controllable via a library and probably has some handshaking and error checking/correction. I'm not entirely sure what the 315/433MHz ones are, they may or may not include these things, or it may be that they are literally a transmitter and modulator, and you have to provide the data that's to be sent. In terms of implementation it might make it more difficult/challenging, and if you have multiple devices you might not be able to distinguish between the two.
The second is more the nature of the EM waves they emit. Generally speaking, lower frequency means lower power, better penetration, and longer range. So the 314MHz/433MHz transmitters may perform better in situations where there isn't a direct line of sight etc. However, an efficient antenna at 2.4GHz is easier to make on a PCB, so performance may be equal.
One last thing is that as you're looking at *cheap* solutions (i.e. not sparkfun branded), the actual implementations may vary. Someone may have a decent NRF24L01 and get great performance, and someone might have a badly made one and find it's not satisfactory. But I guess that's the nice thing about cheap components, if they work then great, if not then you aren't losing much for them to go in the parts drawer and for you to buy something better. For the project you have described however I would imagine that both will perform well.
Good points indeed! I've chosen these two RF links since they are the cheapest, easiest and simplest wireless links that I've found googling about arduino and rf links. There's a bunch of example sketches using 315/433MHz RF links and, some of the examples that I've found for the NRF24L01 show some difficulties with the library, but there are also nice and consolidated projects using this link as posted by innersky.
My main concern regarding my specific project would be:
: at least 50 meters (168 ft) without any obstacles. Not sure if the cheapest version of the NRF24L01 (those without antennas) would cover it.
: RC cars use 2.4GHz between receiver and transmitter. Not sure if the NRF24L01 would interfere with the TX/RX controls of the cars.
: no packet losses when transmitting a very short message once per minute. Well, both seems to attend this demand.
. Both are cheap (less than 7 usd a pair TX/RX)