Re: Superhub + WGR614v9 as wireless access point = N+G at full speed?
You would be best using different SSIDs if only to make it easier to choose between the two when you connect. A dual-band client will simply use the one with the strongest signal, and seeing as 5Ghz signals drop a lot quicker (an extra 6dB path loss in air) it'll probably "prefer" the 2.4Ghz one.
There is no (technical) problem in using the same SSID on 5Ghz as 2.4 or having two 2.4Ghz networks with the same SSID. We (at work) run over 1000 APs on 2.4Ghz and 5Ghz all with the same SSID(s).
Still, your first option would probably be the best, depending on what clients you have. As long as your 802.11n clients are 5Ghz capable, then that solution is ideal. If any of your clients don't support 5Ghz, then they'll be stuck in G mode.
As for annoying your neighbours, running N300 in 2.4Ghz would be far far worse. I would highly recommendatory against using N300 in 2.4Ghz mode. You would not get full speed and would interfere with *every* other network in the area whatever channel it's on.
But ultimately though, you're talking about one printer. One printer isn't going to use the wireless much (even on G, a 10MB document would only cause 5 seconds of interference) and when it's not using the wireless there should be zero mixed mode performance impact.