I'd say Gold is where the meat is and is totally worth it...
Sliver is really a bit of a giveaway level, Marriott gives it away for carrying the credit card. Apart from plenty of verbiage the only real benefit of Silver are the 20% extra points... not shabby mind you!
However Gold brings with it real benefits in particular those sought after room upgrades, lounge access and free breakfast. The room type guarantee and free worldwide wifi completes a really strong package. I vacillate every year between Plat and Gold, without much thought but I make absolutely sure I don't lose Gold!
In my view there are only 2 downsides to the Gold package. Firstly, the extra points are really weak, only a half point extra per $ over silver. Finally, the benefits mean little outside the FS brands, especially Courtyards so it's just as well almost all UK hotels are FS!