Here's one major factor in deciding between focusing on a *A vs a ST program.

If you're a 50,000 mile/year (EQM) flyer, and put that all into a *A program like US, you get TOP-level Alliance status - Star Alliance Gold. That gets you free lounge access on international itineraries and quite a few other benefits of varying worth. For example in US, you'd get US Gold Preferred, which is way done at elite level 3 of 4 within US - but it still makes you top-level Star Gold within the overall alliance.

If you put all 50,000 EQM/year into a ST program, you are LOW-level Alliance status: Skyteam Elite, not Skyteam Elite Plus. So you don't get lounge access, priority baggage handling, or some of the other top-level alliance benefits. ST Elite is somewhat better than Star Alliance Silver (as pointed out by alex0683de, *S doesn't give much at all), since at least it officially gets priority seating and boarding alliance-wide (in theory). But it's still the lower level of alliance status.

By that reasoning, I'd rather have US Gold and nothing else rather than NW Silver and US Silver - if traveling international a lot. However that might be out of reach for you this year. In which case US Silver and NW Silver combined might be your best strategy. If you're flying mostly domestic, then the top-level *A status becomes less important, while the mid-level own-airline status becomes more important (for "home" airline upgrades and other perks). Given that going from 25K to 50K on NW won't get you any better Skyteam status while going from 0K to 25K on US and *A will at least make you "better than dirt" on *A (and will waive 1st bag charges in North America on all 3 *A airlines with the charge: AC, UA, US) then you might as well pick up the *A status.