I signed upto Hattrick, had to wait 4 weeks for a team, by the time I got the team, I just wasnt interested in it anymore and I found the menus utter confusion.
The players are rated by 'good', 'great', 'superstar' and all that rubbish, where as in MZ, they have different skills and have 'balls' in each skill, put a player on tackling, and he will get an extra ball in Tackling in the Training report, which is added to his current attributes. A lot more to do on MZ than Hattrick.
Hattrick also has bog standard tactics, MZ has a tactic page where you have more freedom to place players than in CM4, which means you can be more of a 'Tinkerman' and create the ultimate counter tactics. More depth again.
Hattrick's matches are 'calculated' which means the values of each player is added up and 9/10 the better teams win, no matter what the differential is between the two teams.
Managerzone games are 'Simulated' which is sticking 22 players with AI on a pitch and let them PLAY the game. Which means on a good day, the better team can actually lose. Just like real football.
Hattrick makes their money by selling that HAM program (Hattrick Assistant Manager) which keeps all of your matches and stuff.
ManagerZone has MZManager, which is free and better, and made by fans of the website as well. Due to the better layout and design, and not to mention the paying features, MZ has to make money back, no member is obligated to pay for anything other than the start up fee.
If you dont pay, you can still play in the league, still play friendlies and still build a team and a stadium. If you want to join a cup, challenge a top team/world allstars/national teams/join a friendly league/join a private cup/watch all your matches in the 3d feature, then it will cost you £1.
They are not neccessary, but you do get more out of the game. Hattrick does exactly the same, they just dont have as many features to which you can spend your money on.
If you wanna see a real game that has gone money grabbing mad, then go see Sunday League. They charge you for buying players, selling players, building a stadium and training your players, which is just basically, the person who puts the most money into the game, will win. Which is completely unfair for everyone. MZ does not do the same. The only thing that you can profit from, is when joining a cup, your ranking goes up the more you progress in the cup. And thats about it.