Well here are my thoughts on the choice:
IVAO: Good network stability and good free software. Great coverage over mainland Europe although severely lacking in US. UK cover has been improving lately and I feel it is catching VATSIM up here. Welcoming to new pilots with lots of helpfull members. Lately has been suffering some problems in HQ and Divisions which has provided a lot of political debate on the forums but has not affected the flying or ATC.
VATSIM: Networks can be unreliable in my experience. Rules are more procedurally orientated which means lots of reading and observing is required before taking to the skies. (Follows real world procedures a lot more cloesely than IVAO) Has good cover over US & Great Britian but Europe is no where near as well covered as IVAO. Relies on third party software which is more complicated and is somewhat dated.
I find that Text controlling is far more widespread on VATSIM with more voice control on IVAO.
Of course the above comments are only my opinion and I am sure you will get a lot more responses to this question to help you make your decision, in my opinion the only choice you have to make is where you will be doing the majority of your flying. If it's Europe then go with IVAO, If it's in the US or locally around the UK then go with VATSIM. Both have little cover over the oceans so Transatlantic is pretty much the same on both networks.
***EDIT*** Of course, there is nothing to stop you signing up and using both networks! I do, although I must admit, I use IVAO almost exclusively these days.
