In a regular friendly with no injuries, you can play 11 players and 3 substitutes = 14 players.
In an U21 friendly with no injuries, you can play 7 players and 6 substitutes = 13 players.
As far as total experience points....
regular ones get 150 credits * 6 games per season * 14 players = 12,600 experience points
U21 ones get 135 credits * 6 games per season * 13 players = 10,530 experience points
So a major advantage of regular friendlies is that you get 2070 more experience points per game (spread throughout the players).
An advantage of the U21 games is that all but one of the players only play 45 mins so fatigue is not as much of an issue.
On the whole I think I would rather play regular friendlies as every player can play in them, but only U21 players play in U21 friendlies.
I generally pick the team for my friendlies based on who has games left, rather than try and pick my best team.
Just a reminder that there is another forum with an increasing amount of RS talk:
http://sundayleague.freeforums.org/index.phpIt was originally as a backup for this one in case SL closed down (or if the massive server problems come back) but it seems that the RS talk is the biggest point of it at the moment.