Along with artifacts that grant indestructibility, such as [c]Darksteel Plate[/c], it focuses on cheap green creatures that can add mana to the mana pool
Vesuvan Doppelganger, being a bit more expensive, continues to have the ability to transform into another target creature -- whereas the Clone is just stuck with whatever it turns into the first time
However, it can be offset by using Demon's Roar during the end phase of the opponent's turn
When graded, the value climbs significantly: PSA 9 shadowless examples regularly fetch $225$325 , with PSA 10 Gem Mint versions occasionally selling for $375+
As already mentioned, this card is quite strong for certain decks, making it potentially more popular among competitions than collectors who will certainly prefer the SIR