I'd check out the Tech Support forum for your client.dll problem - run a search for that term first as the issue may already have been addressed in another thread.
As for the path to take?
If you're wanting to work for Zion, thats the course you take as default during your introduction missions. Once those are through, each of the 3 organisations will try to tempt you into working for them, and you'll have the choice of whose missions to run.
To be honest, most people automatically choose Zion. Some because thats what they genuinely want to do, others because at that early stage in the game, they're still slightly bewlidered by everything and just continue with the familiar Zion missions.
This has lead to Zion being extremely over-populated on all servers. While this can have its benefits, ie in pvp etc.. from my experiences, its downfalls are that you end up with a lot of lower level Zionists who quit after a month or two, and a lot of crews/factions that don't last two minutes, whereas most Machinists/Mero's I know tend to stick together and be more cohesive, with a much longer shelf-life.
But... thats just my opinion... its down to you what you decide to do
