first, welcome to FT - my first welcome ever!
second, the answer to your question depends on your location, how convenient NW is for you, etc. but speaking quite generally, yes, Silver is worth it. I value the UGs most, the bonus miles next, and the miscellaneous perks last (pre-board, shorter lines in some instances, etc). From FLL you might find a halfway decent flight to SEA for 6k miles - a worthwhile MR to get you to elite-dom.
unless you have to take substantial MRs every year, silver sounds pretty doable for you, and is much more pleasant than flying with no elite status.