I have always used the pro. version from 6 onwards. The largest difference is the ability to link to and import from external files. For me this is a deal breaker if I could not link up (to Excel dbase, Access tables, SQL, OBDC) Monarch would not help me much. I can use an external table to add value to the reports (like looking up item descriptions)
Also new in version 9 pro is the linking objects allowing you to use anything (filter calculated field, trap specs etc.) from any other existing Monarch model or project file.
I do not recall if the standard version can read pdf files, but the pro version can.
Of course it can everything the std version can plus these 2 huge (maybe 3) improvements, so go for it.