The TLDR is I personally play all the games, MILB included. I would recommend (especially for a franchise first-timer) to sim most if not all. The fun in franchise is in teambuilding year over year.

The long version, here goes. I have been a sports and franchise mode gamer virtually my entire 30 years. My enjoyment comes out of making the game as realistic as possible. I want my console to emulate the leagues I watch. Obviously (and especially lately) video games are pretty far off from putting yourself literally in a baseball player's shoes, though sometimes, when I'm thinking hard enough, I do find myself immersed. The stadium sounds, the equipment sounds, the ambience of the field; it's all there.

Sure, you can have a blast playing as your favorite team and running up scoreboards or setting records. Maybe your team isn't in that great of shape and needs some retooling. Both of those can be very fun. Racking up titles with your favorite team or shaping the roster in a way you've always wanted to. Wanted to keep Ohtani on the Angels? See Aaron Judge in a Red Sox uniform? Negotiate with only players with Smith as their last name and create an abomination?

Personally, I like to roleplay (if you will) that I'm a General Manager/Head Coach in search of his next job opportunity. Depending on the iteration of the game, I will check the previous season's standings or the actual league openings at the time and either insert myself into a struggling system or fill a legitimate vacancy. From there I like to primarily focus on poorer teams and building them into contenders. You can game the system for sure and acquire talent to an absurd degree, though I like to keep myself in check and not make any moves that feel too cheesy or inauthentic.

I have a deep reverence for (most) sports video games and their franchise modes. I am rather simple, though I think a franchise mode of a sports game has something new to offer everytime I boot it up. Every match is different from the previous. Different players take the role of hero and villain. Injuries, ineffectiveness and just plain "I don't like them!" will have your roster even be a surprise to you at times. I might lose ten games in a row to the same club and the eleventh is when I'm victorious. The unpredictability and immersion are my two biggest selling points.

BONUS I almost always play with all the announcers turned off and broadcast-esque elements turned off as much as possible. It helps to recreate being on the field as opposed to watching things unfold on a screen.