Version 7 is really different from version 6. I think it's probably the biggest change in FM history, at least recent history.
In previous versions you had one table per file. You could relate data from one file to the next but scripts were not really able to be shared. This means that there is a lot of duplication of scripts (effort) when you get more than a couple files (tables) involved in your application.
Security is 500% better in FM7. In previous versions it was pretty hokie but with 7 its what you would expect to see.
DB capacities have been greatly increased too with v7.
If you are just getting started on developing in FM I wouldn't even consider an earlier version. If you have a lot of stuff already built, that's another story.
At the end-of-the-day, you have to follow the crowd eventually but it's up to you to determine how far back you're willing to travel.
-Striker