Solidity makes no difference to the poly count at all. It does, however, change the node count (or BSP cuts).
Solids make BSP cuts and are the most common type (ALL subtractive brushs are always solid).
Semisolids don't make BSP cuts to the surrounding area leading to a lower node count. There are a few side effects however, such as collision errors, sometimes bot problems, and if you have a semisolid brush, I personally find you are more likly to get BSP errors on that particular brush than if you make it solid.
They also do not mix sell with zone portals, non-solids, or bots
Non-solids are as semi-solids except the player can walk though them. Sheets are ALWAYS non-solid, even if you set them as solid or semi, they will always be treated as non-solid as the player can always pass though them.
Coronas, and I think sprites also, can be seen through non-solids (so they make good hanging lanterns). Bots can see through them.
They so not mix well with semi-solids and I find can frequently cause nasty errors even though they don't mak bsp cuts to the rest of the map.
The reason I said semis are best for any brush with a dinension of <8 is because semis don't make bsp cuts to the surrounding area and brushes that are <8 units can start to cause bsp problems.
I don't know if making brushes semi-solid reduces the map size, It might slightly if there are less bsp cuts, but I really doubt it's worth the risk of errors.
If you really want to keep the size of the file down, do your final rebuild, then delete all the brushes. (always keep a backup with the brushes not deleted for obvious reasons

You will still be able to play the map etc, but you won't be able to edit it at all in the editor as if you rebuild the geometry, the whole level will dissapear.
Enough talk for now. I can't even remember what this post was origionally about lol
