What's the difference?
Construction paper:Construction paper is a very soft, absorbent paper that's much like newsprint in the way it takes wet media. It soaks up ink and paint like a sponge. Blobs and splops are often the result, making it hard to control the application of the ink or paint.
Also, construction paper fades incredibly fast. Leave a sheet of it in a patch of sunlight for a day or two and you'll see a big difference when you compare it to another sheet that wasn't exposed to UV rays. The paper that was exposed to the sun will be noticeably paler in color and washed out looking. Construction paper is very high in acid content and is very "pulpy" which is why it degrades so quickly.
On the plus side, it comes in a wide range of colors. It's easy to crease and fold, making it ideal for children to use. And it's usually inexpensive even when it's not on sale.
Card stock:Card stock is much sturdier than construction paper. It retains its color much better over time, although -- like all papers -- it will eventually fade when exposed to UV rays if it's not sealed and varnished with a UV protectant. Even unsealed it takes card stock much longer than construction paper for its color to start looking pallid.
When inks or paints are applied to card stock they tend to remain where you want them to, depending on the quality of the paper. My recommendation is to purchase acid free card stock whenever possible. If you're making handmade greeting cards or other paper projects that might be cherished by their recipients, using acid free card stock will result in something that's likely to look good for years.
Card stock doesn't fold quite as readily as construction paper does. Using a bone folder on it results in a nice crisp fold. Bone folders are readily available through arts and crafts stores and Internet sellers.
Card stock might cost just a wee bit more than construction paper but in my opinion paying the slight difference is worth it. Stock up on a range of colors when you catch it on sale. You're sure to find plenty of uses for it!