STO definitely has a larger playerbase than CO. I can tell by the queue times, Zone chat, how fast my stuff sells in the market, the number of Fleets recruiting (of all sizes..) and the amount of resources I see pumped into Fleets. I'm getting a feel for Fleets anyway, here, in my second Fleet - Go go Wolf Pack! Arrewwwww!
CO has an international playerbase but STO's is internationallier - for trivia's sake. I would rate the *average* age at around 3 to 400 years older, too.
On a scale of 1 - 10 I would rate STO's playerbase "strength" (I'll post my Zone Chat Mids when it's complete) at ~ 9.5.
CO has a strong playerbase, too, not putting it down, just using it for comparison since that's what I was playing.