I can´t judge any before launch, but comments in ˝symmetrical˝ thread on Kronos seem to be much less tolerant towards Nostalrius (that includes some devs and moderators) than vice versa , and in general if forums were good indicator of community, I´d pick Nostalrius without blinking an eye. Of course, I can be wrong on that. Also, Nostalrius is ATM more open for us to personally test it.
On the other side, Kronos is much more visible everywhere (I hope this will change as we get closer to Nos launch) and it could easily start with bigger/much bigger population, especially if it launches first. If their scripts and level of polish are anywhere close to what they are saying, it should be very good.