There is actually not much of a difference between a solid spruce and a Sitka spruce. In many cases a product which is labelled as being made out of solid spruce will actually be Sitka spruce.
A Sitka spruce is simply a particular type of spruce that can be found in North America. It can also be known as Norway spruce, this is the wood that is used to make most guitar tops, although it may or may not be advertised as so.
However the wood that comes from both types of spruce are pretty much the same, or will at least perform the same in terms of strength.
If a product you are looking to buy is labelled 'Sitka spruce', this is likely just a marketing technique that made the item seem more appealing to the buyer.
The term 'solid spruce' is actually just the common name for over 30 different types of spruce, including the Sitka, so really they could be the same thing...