I have personally experienced 3G being faster than WIFI, hence in answering the OP's original question, my answer is still "yes".
Your answer only takes half of the process into account.
Yes, a wifi will be slower if there is a lot of traffic or if you are so far from the router that you're dropping more packets than you are receiving.
You ignore the fact that 3G will suffer exactly the same problems for exactly the same reasons. If you're in a concrete basement, far from a cell tower, or are getting a bad signal for one of many reasons, 3G will be slower.
Since there are a lot of very obvious factors that could slow down a wireless signal (whether wifi or 3G), a discussion like this mentions them once and leaves it alone. Arguing about it is silly.
What is important is that 3G is much slower than wifi under ideal conditions. Much slower. Add to that the fact that 'ideal conditions' for your wifi are much easier to manipulate than 3G ... you can move your router, use an antenna, boost your signal, etc. but you can't alter the signal from the cell tower ... and you can see that wifi is a 'better' connection than 3G in most conditions.