If you had experiences regarding flying virtually on either VATSIM or IVAO, what is your personal view of the difference of these two FS online flying communities?

It's the Coke vs Pepsi question. I will give my personal opinion. VATSIM is ATC oriented. IVAO is a little more flexible on this aspect. Being an ATC on VATSIM it's quite hard and tough. The standards are very high, so every ATC is as real as you can get. On IVAO you can find some top quality ATC service mixed with a more colloquial ATC. VATSIM has more presence on the US, South America, Oceania, UK and Germany. On the other hand, IVAO wins on southern Europe, Italy, France and Spain. It has a lot of Indian and Brazilian pilots too. IVAO has practically no presence on the US. I disagree with Roger that VATSIM ATC tend to be hard on newbies. I started flying on VATSIM when I was a kid and I've made lots of mistakes and never have had an issue with any controller, they were very polite and comprehensive. But it is true that they are demanding. The last thing to address is the pilot client*. IVAO has it's own pilot client called IVAP, it's an excellent software, here IVAO defeats VATSIM for now ( a new pilot client on VATSIM is being developed). The disadvantage of IVAO is that they use an external client called Teamspeak for voice communication. I prefer the all-in-one VATSIM way. VATSIM has three different pilot clients. Two of them (Squawkbox and FSInn) are discontinued and frankly very outdated. vPilot, the most recent one, it's excellent, the closest thing that you could get to match IVAP. Things that I would consider to choose between these two communities: a). Where do you like flying. b). The help and tutorials available c). The level of "professionalism" that you wish to reach.   *pilot client: The software you need to connect to the network and fly!.