Oh wow, this IS a hot take.
I like to think of the Jazz Messengers as being an incubator for young talent, but with the musicians playing within the confines of Blakey's ideas. He allowed for plenty of freedom within the solo's and compositions but at the end of the day, it was down to him which tunes got played and how the musicians were to approach them. What it did was allow each individual musician time to really focus on their playing and writing because they didn't have to lead the session and worry about everyone else, which would naturally create a more polished result.
By the time Morgan joined the Jazz Messengers he had already led 7 sessions (including big hitters such as The Cooker, City Lights, and Volume 3) but it was AFTER his time with the Messengers (he was forced out in 1961) that his most successful albums (the Sidewinder, Search For The New Land, The Rumproller, Cornbread) were recorded. It could be seen as a natural progression but its clear that he benefitted from his time with the group.
In terms of comparing the 2, I personally prefer his time as lead as we really get to hear how he imagined all of the musicians should sound within the context of the performance (whereas in the Messengers his job typically was to follow Blakey's decisions and/or solo). He definitely cooks in the Messengers and his time with the group is still my favourite of their iterations.
The Sidewinder was Blue Notes most commercially successful release and it's hard to deny that the title track is super catchy, but I agee with you that there are others that are better. I much prefer Search For The New Land, Volume 3 (Hasaan's Dream alone is worth it) and The Cooker.
