Hi,
There is nothing much difference between the webi report and bw report. You can do pretty much the same calculation and stuffs more into BEx query compared to webi.
I would suggested you to go with "Query browser" connection where you can able to connect the query directly to the dashboard, when it comes to performance its much better compared to other connections.
Only pain point of using query browser you cannot your report as cross tab, where you can design or incorporate the report into the spreadsheet in horizontal or vertical. Apart from this if you still interested in using the webi between the dashboard and Bex query, I would you to go through the below links which can provide you some better clarifications on this.
Major Difference between Qaaws, BI services and Live office
BOBJ Tricks: Live Office vs. QaaWS vs. BI Services
Also, do you know if there is now a way to publish a Dashboard in BusinessObject Platform if the Dashboard use a direct connection to BW?
If you are going to use SAP BW Netweaver connection then you can publish the dashboard only in portal, using query browser you can publish it to BOE server.
Hope this helps!!!
--SumanT