If you only have one Tivo, already have a G adapter and don't have to or want to invest any more money, then I'd just use the G adapter. But...if you plan to stream netflix, hulu or some other on-demand service, the G might be problem if you don't have great signal strength or have multiple active wireless devices. If you get another Tivo, not sure G will stream HD very reliably. If you decide to do something else, +1 on the MoCA.
3 Premieres w/2TB each