Do you already have or have the ability to get v11.0 of the product? Since 11.5 has been out for over a year, I suspect that you can't purchase v11.0 anymore. So, your question may be moot.
The pro of going with v11.5
There is more functionality available in that version than in 11.0.
Support for future browsers and Windows OS versions for both client and server systems is being implemented only in 11.5 and beyond. For instance, HP has no plans to support Windows 8 and beyond for any release older than 11.5. Unfortunately Windows 8 isn't yet supported at all, even in v11.5.
The pro of going with v11.5
There is more functionality available in that version than in 11.0.
Support for future browsers and Windows OS versions for both client and server systems is being implemented only in 11.5 and beyond. For instance, HP has no plans to support Windows 8 and beyond for any release older than 11.5. Unfortunately Windows 8 isn't yet supported at all, even in v11.5.