First off, there's a very good (though no longer guaranteed) chance that the two servers will merge within a year. See
https://wiki.project1999.com/Green#How_Will_it_Work.3F
Second, both servers are all but identical at this point, code-wise. Both are set at the end of Velious, just before Luclin (and neither will ever reach Luclin).
Finally, both servers have healthy populations, including plenty of people leveling up alts (ie. there are potential groups at all levels on both servers). Both servers also have guilds welcoming people, at all levels of the game.
So where does that leave you? Basically comparing on population/economy. Blue, being the older and "beta" server, has far more of many items in its economy, meaning those items will be cheaper for you to acquire. In general players on Blue will have better gear, and you're more likely to be given nicer gifts on Blue.
But, Green's economy is closer to live's (ie. "real" EQ) because it has only existed for a bit over two years. So if you want to avoid "twinks", and players with a million alts, Green is probably a better bet (although it does have some twinks/many alt players also).
Again, there's really no bad choice, and in a year it might not even matter which you pick.