I suppose I'll throw my grain of salt into the pile. I've given each method at least a shot and I have found each to appeal to a different desire of mine. I've been on a fictional kick lately because I dug up an old save file from OOTP 18 with a pitcher who I can say I'm more interested in than IRL players. While in this file I play GM only, I will at some point transition to GM/Manager to give myself more attachment to my players. I would encourage everyone to at least try playing out every game of a season, minor leagues or majors, to see just how attached you can get to your players. Plus it's nice to sit up in the box seats (or anywhere in the stands) and watch your little players do their things.
That being said, I also have a manager only save file going where I took the reigns of the Detroit Tigers and plan to stay with the franchise for as long as I can, through bad times and good. While I'm only midway through my first season in this file, I've certainly grown attached to my (fairly bad) squad and had a few choice words with my fictional GM when someone I liked got traded away. Some days I will play every pitch and other days I will use the one pitch option but I make it a point to play every game. When you combine this with creating a camera angle from the dugout it's quite the experience.
I have tried historical and random historical and I plan to revisit them again but thus far I think my main attraction is either fictional, real-life, or a hybrid of those two. Perhaps in my next attempt at historical I'll play manager only so that I have to make the best of the players that I've been given whether it's Honus Wagner or Mario Mendoza.
On another note, on a whim I tried playing a manager only mode as a minor league manager and as with detroit I started caring more for these players than their IRL counterparts. Of course, I will add that the broadcasting in OOTP being much more tolerable and much higher quality than the broadcasting of IRL games, radio or televised, plays a small factor in this.
In the end, the fact that we and I can have all of these various playstyles and actually enjoy them speaks volumes to what the crew here have accomplished.