Okay, now for a newbie question: How is illustration board different than bristol board? I use bristol board for my colored pencils and when I have used watercolored pencils on the bristol board, it didn't like the water much. I could only used a little before the surface of the paper turned ugly. So what is illustration board?
Illustration board comes in hot or cold press (smooth finish or "toothy" or textured finish).
Its usually a piece of bristol-type paper adhered to a chipboard backing.
They make un-archival and archival boards, in different weights, and some are usable on both sides.
I use Strathmore 500 Series boards, which are archival, and both the front and back are the same. Its a very good quality board.
There are also some differences in how "white" the surfaces are (like watercolor paper). Some are warmer white, some brighter.
There are differences by manufacturer, and lots of options. Illustration board is what we used all through art school (Crescent 300, cold press), and it stuck I guess. I did gouache, watercolor, acrylic - everything on that stuff! I just love that it doesn't warp. But having said that, I have a more "render-y" style of painting, which is more picky and detailed. I don't do big washes of watercolor, or need any watercolor 'effects', which watercolor paper would be better for.
It all comes down to personal taste. I would get a few pieces of board to try them out. Most art stores sell pre-cut pieces (15 x 20 or so), or have scraps they'd be willing to sell, so you wouldn't have to spend a lot. The only way to know what you like is to try new stuff!