Alginate is bar far cheaper to use. I'd say if you can lifecast with one you should have no issue with the other. Silicone offers the advantage of being able to make many casts from the silicone mold. Aliginate dries and shrinks pretty quickly so you will only have one cast from it. If you need that ability or the material is moisture sensitive (like polyurethane) then silicone is the way to go. One nice thing about silicone is that it sticks to itself so you can apply a thin detail coat over the model and then build up a layer or two of thickness in separate batches. The last batch you lay down premade silicone keys for the mother mold to key into. On the other hand it does take a bit longer so if your model wants out quickly then it may not be the best direction. You could always do an alginate mold and create a master silicone negative later from the cast. As for storage if it's a cured platinum silicone (it would be for lifecasting) then it should last for years in storage and it won't shrink (this will vary depending on how many casts get pulled from the mold and what chemicals are used). If it's the separate uncured silicone parts then officially it's a 6 month storage however it should work fine even after that. You just have to watch out as it will thicken up over time making it more difficult to work with.