I have the 9 and don't plan to switch to 45. Nothing against 45 but I'd prefer the extra capacity of the 9 and I don't think that the 45 offers the advantages in terminal ballistics that some ascribe to, particularly in short barreled pistols.

Even though I'm fine with the function of my 9 Shield, ultratec00 makes some good points specifically about the 45 Shield that seem worth considering.