Well, Gold is (in general) better than +1 Card, +1 Action, +$1, which is similar to the coin, so disregarding the super-availability the coins have, one-shot Gold should be better than one-shot peddler. But, availability at any time is a HUGE deal. The spoils may never be seen again, or may be seen in a hand of $9 or more, so IMO they are in general not better than the coin.
When greening, I would spend the spoils even if I need only $1 more for Province (assuming a BM-type deck in which you reshuffle in 4 or 5 turns time). For Colonies, I don't know. Colonies are really expensive and $1 is much less for Colonies and the +$3 from Spoils, thought swingy, can actually make a significant difference. Also, how far is the reshuffle should be considered (in the category of "how likely am I to see the Spoils again while its still significant to do so").
When not-greening the question is quite different. If you are building an engine and you expect to have good +Buy that you don't currently have, maybe you want to save the spoils for the time in which you can get something out of the $3 it offers. In this case it would be even more difficult to make a broad statement.