Reliability is first and foremost in a defensive handgun. The 9mm shield has been around awhile and proven whereas the .45 ACP version is much too new to make that declaration.
I would also assert that 9mm's are generally more reliable overall than .45's, especially in smaller weapons. Just my opinion based on observation, experience and input from others over the years rather than based on hard evidence and stats, so take that what's it's worth. Research it and see what you come up with.
And "shoot better" can mean very different things depending on who you ask. It should be in context of what you would likely have to do in an actual defensive scenario rather than static range work. For me, it essentially means being able to dynamically access the weapon and reliably put multiple rounds on target quickly all while in movement, including one handed firing at relatively close range. Nearly everyone can do that better with a 9mm then they can with a .45 ACP in equivelant sized guns and similar platforms.