When I started with SOM online--I think about 15 years ago--I played the Mystery Card game exclusively. At first there was only Back to the 80s. Then came the 70, the 90s, and later the 60s. I liked the day-by-day involvement in the games, trying to figure out which year I and my opponents had, etc.
Then I started trying out ATG and eventually got pulled into it--partly because the ATG game kept expanding and changing, with new players, new parks, new pricing--all of which tend to lead to new strategies. Also, the game can be really interesting to play at a wide range of salary levels, from 60M to 200M and beyond. Each salary level calls for a different approach, and leagues can involve a battle between a range of team design strategies.
Now, I play the Mystery Card games very rarely, and with the arrival of ATG9, I may stick with this format for good.
If you're going to form a league with 12 Mystery Card friends, why not try a 100M league rather than 80M. That will give you a a slightly different look than one gets in an 80M mystery league, without being too radically different. Why not give yourself enough salary, however, to be able to play a couple of superstars such as Babe Ruth or Ted Williams. DH or no DH should be your choice. Personally, I like the DH, but opinions vary.
You can look at varied team designs by exploring the ATG 2020 Barnstormer Tournament, when all the players are learning the ATG9 game on the fly:
https://365.strat-o-matic.com/tournament/37. Managers were allowed to chose their preferred salary levels, so we've got all kinds of different games going on right now, with everyone in a process of experimentation. I'd say, dive in and have fun!
Also, check out the Diamond Dope website. This provides information on every card in every park in the ATG9 game. There are more than 5400 cards to choose from, so it's good to have a little help:
http://www.diamonddope.com/card_reader.cfm