Letterheads, continuation sheets and compliment slips are often printed on 100gm white cartridge paper as it is designed specifically designed to perform through laser and inkjet printers.  

There are many other types of paper to consider, such as woven, watermarked, laid weave, as well as coloured paper.

When considering new letterheads, and compliment slips, don't forget to get matching envelopes to complement your Letterheads.