Going by the sales figures on John Jackson Miller's Comics Chronicles, IDW has done three (completed) tNG
miniseries (The Space Between
, Intelligence Gathering
, and The Last Generation
), the average issue of which has sold 8,800 copies. (This will go up once Countdown
is factored in, though.)
They've done five (completed) original series miniseries (Klingons: Blood Will Tell
, Year Four
, Year Four: The Enterprise Experiment
, Mirror Images
, and Romulans: The Hollow Crown
), the average issue of which has sold about 9,600 copies.
So apparently the original series sells slightly better, though I suspect once Countdown
is factored into tNG
they'll sell pretty much equivalently.