Play money is fine for learning the rules and basic fundamentals. Just don't expect it to play like real-money games above the penny stakes.
Don't confuse public play money tables with Home Games for play money. Home Games are created and run by players who control who the participants are, so obviously you can find some very challenging games there. For example our CardsChat league is run there, and it's some of the most challenging play you'll find. But the public games are by and large not going to be taken seriously by most players, which makes it more difficult to get the same feel as a "real" poker game
. Something has to be on the line (money, prizes, bragging rights, etc.) for most people to be expected to play somewhat seriously.
As to .com versus .net, definitely get the .com client. US players can still use the .com client. Not only can you rail real money games, but you still get HH's from your play money games (useful in serious Home Games, especially League), replayer, avatars, etc. that are disabled in the .net client.