Tenfoot/Jeff,
I would say that Studio One covers 90% of your midi needs if you are primarily ITB. The missing 10% can be covered by adding a plugin (for step sequencing, if you are into that sort of thing) or doing without (Cubase expression maps). We didn't have expression maps in Sonar and we did just fine.
If you have
external synths, then it is very convenient to be able to access and control them using all of their parameters, and Cakewalk made this very easy through Instrument Definitions. I don't know what other DAW can do this in a similar manner, with the same functionality. Perhaps Cubase?
On the other hand, if you can do what you need in Studio One, it is a whole lot more streamlined to use than Cubase. In the long run, and in daily use, that counts for a lot in my book.
Regards,
Dan