I've got very little play experience, so my gut feeling was that they're roughly the same strength, although obviously good in different contexts. An early silver will boost your economy for the entire game, so silver is probably better early (although, Spoils does get you to better cards earlier...), but later when greening, you're probably only going through your deck once or twice more, so a Spoils would probably be more useful.
I think, actually, spoils are probably going to be usually better, especially if you want to build an engine (cash injection then nothing, instead of a persistent extra card)