Is wireless n only faster than mixed mode?

Yes, 802.11n-only (also known as greenfield mode) will operate much faster than mixed-mode 802.11b/g/n. When in greenfield mode, the devices will all operate at the higher rates of 802.11n without any errors or backoffs. In mixed-mode 802.11b/g/n, your 802.11n community of devices will be running in greenfield mode, until an 802.11b or an 802.11g packets is heard by your AP or devices. In that case, since 802.11b and 802.11g traffic runs at a slower rate, it will need more air-time to transmit. In order for the slower stations to get their packets through, they send an IE (Information Element) in their packets telling the AP and devices to hold their transmission for the duration of the timeout, so that the 802.11b and 802.11g devices have a chance to transmit. As you can see, this will affect the performance of 802.11n by throttling its transmission to allow for slower traffic to get their chance at transmitting packets. As far as more reliable, in a sense yes - Running in greenfield mode will allow for less errors and collision of packets, but we all know that TCP is resilient and will retry.

N-only (greenfield mode) is only faster when your radio environment is like a "green field". That is, when there are no G or B devices in radio range of your network, and when your devices are close enough to the AP to use N rates.

N rates use modulation and coding schemes that are too complex to decode reliably at low signal levels, so even N clients fall back to G or B at range when needed, if the AP allows those rates. So disabling G and B shrinks the usable range of your network, and doesn't help much unless you live out in the countryside without any neighbors with old equipment.