I think there are a few key differences between the LC9s and the Shield...some of which may matter to you, and some of which may not. But it's important to acknowledge them either way.
Safety: the Shield comes either with a very flush safety or now also completely without the safety (two different models). The LC9s comes only with a safety, and the safety lever has a much thicker profile than that on the Shield. Keep this in mind if you either do or do not like manual thumb safeties. The Shield will appeal to you if you like a more traditional striker-fired design and do not use a safety and the LC9s will appeal to you if you prefer a manual safety.
Weight: the Shield weighs something like 22 or 24 oz empty, vs. about 17 oz for the LC9s. The Shield will very likely shoot flatter and with less of a recoil impulse. It's commonly said among Shield owners that it's one of the few, if not the only, "compact" gun that shoots like a full-size. Owning one myself (a Shield), I can attest to this. It shoots extremely nice for being so small. The relatively hefty weight likely contributes to that. If you prefer the lightest weight possible, the LC9s has a decided advantage.
Magazine disconnect: the LC9s has it and non-CA and non-MA models of the Shield do not. This is a deal-breaker, or a deal-maker, for some, depending on your preference.
Materials: the Ruger is listed as having a blued slide and barrel. The Shield has a stainless steel slide and barrel, and both are treated to Smith's Melonite coating, which I can confirm is extremely resistant to wear.
If anyone is interested in the Shield, including the OP, be aware that there are many variants offered by Smith, to cater to individual markets. There are models with the thumb safety and models with no thumb safety. There are models with a heavier trigger for the MA market. There are models with a tactile loaded chamber indicator and magazine disconnect for the CA market. Local gun shops sometimes get whatever they can get (it's a popular pistol), and you may actually find a MA- or CA-compliant model in a gun store in, say, Kansas, where none of those regs apply. So if you don't like the MA- or CA-compliant features, make sure that the SKU you're buying is what you want (they're all listed on Smith's website).
The Ruger appears to not offer a MA- or CA-compliant LC9s, so that situation wouldn't apply to the Ruger.