Listening to simulcast systems can be challenging.
Have you got the latest firmware updates in your scanner - some folks have found that the firmware upgrades have improved the simulcast issue. I did see an improvement with the 396XT on our local simulcast system, but minimal improvement with the 996XT after the firmware upgrade. So, like you, my 396XT appears to be "better" than the 996XT.
As far as the P25 settings go, I suspect a lot of what the "best" settings have lots of variables, even your location with respect to the transmitters. I have my settings at 13/175 and get tolerable reception, but a friend with the same model radio listening to the same system uses different settings and swears by them.
So the bottom line is this - keep experimenting with your settings and see if you can come up with a good combination. Additionally you might try setting the site attenuator to "On". The problem we have is that we're hearing multiple sites - the radios are receiving "too good" and that causes either distorted or absent audio. In some cases, having a "lousier" antenna setup can be helpful - there is a post about using a bent paperclip for an antenna on a simulcast system with good success.