Many people see cash advances and balance transfer checks as one in the same: an easy way to draw money from your credit card account when you need it most. In reality, a cash advance is a very different offer from a balance transfer.
A balance transfer is almost always:
Lower interest than a cash advance - Balance transfer offers tend to have much lower rates of interest than cash advance offers because of the difference in the offer. Balance transfers are not meant for emergency funding, but for somewhat longer-term financing than a cash advance.
Lower fee than a cash advance - Cash advance fees run as high as 5-10% of the total amount that you take as a cash advance. A balance transfer, by comparison, usually has a fee of 0-5% of the total balance, and comes with much lower interest rates over the long-term.
Better for your credit standing - The simple fact is that while credit card companies do like making cash advances to customers (that's how they make money), they would much prefer to see customers pay on time and use financing offers that do not imply financial trouble on behalf of the borrower.
When in doubt, always seek balance transfer checks over a cash advance check. A balance transfer check is much less expensive, far better for your relationship with your credit card company, and a better method of financing when you need cash now. Most credit card companies have a long list of balance transfer offers available at any one time, meaning that you can access a balance transfer just by logging into your online account.
Additionally, balance transfers offer much larger total financing amounts. In general, a cash advance tops out at $400, whereas you can make balance transfers up to the entirety of your credit line.