well I play both of them...
It depends how much time do you have to spare
The best choice is to play both
but MZ takes a lot more time than HT - you play many more matches and it is connected with more fun than HT, but HT is more realistic beacuse your players do not play 3 matches at the same time at different places like it is possible in MZ.
You can log into hattrick only 4 times a week (If you do not make transfers - on sunday: to watch a match or check result, also to check your youth, on tuesday: to change the tactic for friendly (playing with B team), on wednesday watch a friendly, and on friday to see how your training was and set tactic for league).
Hattrick has a lot worse graphics but for me that doesn`t matter.
Training in HT is rather poor - only those who play in match train but you can train one type of training during week - so if you train keepers rest of your players doesn`t train at all.
HT has something that MZ do not have - stats of your formations - so you can check which part of your team play OK and which rather poor. Also MZ is more random (e.g. there is sometimes a bug wuth friendlies that you play more than one at the same time and place - then you can see that with the same tactic 4 matches ends with totally different results like: 5-0, 2-0, 0-0 and 1-2 - so you really do not know how good your team is)
Also there are some interestiong addictions to HT this season which may change the view at tactic.
I liike both games - Just make a team in both games and see which is better for you