Sorry in advance if this sounds like LC9 bashing but I just feel Ruger did not do a good job developing the LC9.
I have both the LC9 (original) and the Shield. I bought the LC9 first.
Speaking only of the original LC9 version in this case, I prefer the Shield.
Two problems with the LC9 that bothered me was the ease with which the magazine can be pulled out of the locked position within the grip with pressure on the magazine base pad, and the fact it's blued steel as opposed to something more corrosion resistant. I found mine had developed significant red rust between barrel and slide and in the front sight dovetail after several weeks of carrying in a leather IWB in the summer heat.
The LC9 has a magazine disconnect safety which I feel has no place on a defensive handgun. The Shield does not.
To field strip the LC9 you end up with a free floating pin to keep track of and not lose. No such loose parts with the Shield.
I can shoot the Shield almost as well as I can a full size pistol. With the 8 round mag in the handle I have a full grip. I've never shot a group with the LC9 I would be willing to share with the public.
The Shield is Melonite coated stainless and not likely to exhibit any sort of corrosion even under the most adverse storage or exposure conditions.
I was hoping when Ruger re-released the LC9 they would address some of the liabilities in the original version. It does appear Ruger addressed the trigger, the "shark fin" loaded chamber indicator, and removed the completely useless internal lock, so that may improve its standing among the other offerings.