On the small to mid-size trimmers, go with the Speed Feed 400. I run it on my FS94R, FS74R, and my FS111R. I run the larger 450 Speed Feed on my FS240R.
I think the 400 allows for greater head speed and more manueverability than the 450, which is a benefit on a smaller displacement trimmer.
I don't have a problem with the standard 25-2 head that comes on the Stihl commercial trimmers, but time is money when I'm running a trimmer. Seconds equal minutes equal hours equal time and money lost on my jobs.