Silver proof sets, clad proof sets, or just mint sets?

The US Mint sells mint coin year sets by assembling all the coin variations issued during the year, packing them into plastic storage boxes (previously cellophane pockets), and charging a significant premium over the face values.  The mint sets are just new (a.k.a uncirculated) common coins, usually with P and D mint marks.  The coins in the proof sets are struck with higher pressure under a different manufacturing process, and they show much more details and brilliance.  So they are visually of higher quality and, not surprisingly, more expensive than the mint sets.  The silver version of the proof set is the most expensive of the three.  However, not all coins in the silver proof set are made of silver.  For example, in the recent years, the dollars and cents are made with the same materials as the clad proof sets.

Intuitively, silver coins are more value-preserving than the ordinary coins, and hence should be a better investment option.  The pricing data published by the well-respected United States Coins (the "red book") supports this hypothesis.  Uncirculated mint sets from 80's and 90's mostly have a reselling price well below the issuing price.  The situation improves only for those issued after 2000, mainly due to smaller mintage.  The observation is similar for clad proof sets, although the proof sets are of higher quality and supposedly lower mintage.  The silver proof sets, on the other hand, have shown strong performance consistently throughout the issuing history.  Many of them have the current reselling prices doubling or tripling the issuing prices.  Hence, though the silver proof sets are significantly more expensive to acquire, they are indeed better candidates for value appreciation.  However, if one does not care about value but just wants to collect the year sets themselves, the mint sets are a much more economical alternative.