I don't think you will have any luck connecting Crystal to OneSite. Unless RealPage has changed their policy they do not allow you to use ODBC to their hosted database. They do use MS SQL, but as far as I know they will not allow you the access you would need. Contact customer support to be certain.
RealPage does sell a reporting tool/ package but you will pay a fee for it and, I think, the access it not real time. Not that thst is usually much of an issue as you typically would report off a snapshot of your data taken at night.
Yardi is different, assuming you host the database. If you do, then Crystal can be used to report off the database which will be either Oracle or MS SQL. Given the choice, IMO you are better off going with SQL as Yardi develops first on that platform and later to Oracle.
Feel free to contact me if you have any specific questions on the systems.
Don
www.dbrw.com