This is a completely untrue. It is obvious to me that the contractor is either uninformed or trying to convince you to buy the more expensive cable to benefit his own agenda.
You should stick with the lower priced CAT5e. Cat5e works great for existing 100BaseT networks and in fact was designed to support Gigabit Ethernet (GE). It is true that CAT6 is better cable, however, the TIA/EIA spec has not been finalized yet. CAT6 is more expensive and comes with a lot of vendor hype. In reality, CAT6 won?t buy much for you. The electrical characteristics are better for increasing signal strength while reducing noise within the cable. However, the advantage for this may never be realized within the scope of today?s data applications. The best cable you can buy for the money is CAT5e. Here are the differences:
The different categories defined by the TIA/EIA depict the amount of attenuated signal loss versus the internal noise (crosstalk) between the twisted pairs of the cable. This is known as ACR (Attenuation to Crosstalk Ratio). Adjustments to these variables are done by thickening the actual copper and by varying the twist ratios of each pair. Cat6 uses 23AWG copper as opposed to 24AWG used in Cat5e, therefore the signal attenuates less with speed and distance. Cat6 also uses tighter twist ratios that cut down on internal crosstalk. The theory is that if your signal is stronger and there is less noise you can achieve greater ACR and therefore push more data through your cable.
While this is true, keep in mind what your objectives are here. GE was already designed for and works great over Cat5e. By the time you need to provide bandwidth above GE, you will probably be considering fiber to the desktop.
One other issue against Cat6, is that since there is such a difference between the loosest and tightest pairs, the skew delay (difference in the time it takes the signal from the loosest pair and the tightest pair to reach it?s destination) can cause some applications, such as Ethernet to time out. Cat5e doesn?t have this problem because the skew delay is typically 20ns, while Cat6 is typically 38ns. 38ns is right on the border of the ethernet spec of (40ns) before timeout. With Cat6 your longer cable runs may timeout.
Having said all this, I hope it clears up some of the confusion and hype about these two cabling technologies. Therefore, it is my opinion and recommendation that you buy 5e for your cabling needs.