VATSIM is usually much less busier than IVAO, also they are concentrated mostly on the US. However, they have just released a new pilot client, so traffic and ATC number is very high now, as everyone is trying the new software. I guess after a couple of weeks, the number of people flying on VATSIM will decrease, although it will be still higher than the levels before.
Both networks are an environment for flying with ATC, but the experience, the athmosphere differs. Some claim VATSIM is more professional, however not many people can confirm this. If you like, try both networks, and decide which one is for you - or fly on both, depending on ATC coverage.
VATSIM does not have an awards/promotion structure, except the upgrades for controllers (4 controller and 2 instructor ratings).