I don't know enough about SkyTeam or WorldPerks to comment on them, but I can tell you that *A Silver is pretty much worthless. You're not even officially entitled to priority check-in though many agents will let you get away with it.

The only official, listed benefits of *A Silver are:

* Priority Reservations Waitlisting - when there aren't any seats left on your preferred flight, gives you priority should a seat become available.

* Priority Airport Standby - gives you top priority when standing by for a flight at the airport.

(from http://www.staralliance.com/en/trave...er_status.html)

I've heard that US Silvers get an upgrade on US once in a while, but you don't seem to fly with them very much anyway if you say you mostly get your miles from Lufthansa. To Lufthansa, you will be on the lowest rung of their status totem pole, which goes Miles&More HON Circle (basically *G on speed), Miles&More Senator (*G, with 100K status miles in a calendar year, at least every other year), Miles&More Frequent Traveller (*S, with 35K per year), other *Gold, other *S. There have even been reports of them preferring non-status members of Miles&More to non-LH Golds or Silvers.

Status is almost always worth most on the carrier who owns the programme. Since you actually do fly Northwest, I'd concentrate on that.