The ST is a better games machine, dont ask why. This is just my opinion, and I have used A500/600/1200 to compare against. I just get more excitement from the ST.
Workbench is a nicer opersting system though, TOS isnt very pleasant. The only good thing about it is the nicer background and the fact that it loads from chip and not a disk although it takes a while.
But I solved this problem - by simply inserting a disk on startup with no data on it.
It makes it boot straight into TOS instead of waiting around 30 seconds watching a white screen. :-)