If I need (or want) more elevation than the scope provides, which option is better? Use a 20 MOA base, or get adjustable rings like the Barrett EXRings? What is more stable?
Get a canted scope base and skip the adjustable rings. While both will work, the adjustable rings just introduce another variable into the equation and the potential for something to come lose and cause you problems.
Another vote for base. After mounting there should be as little space between scope and barrel as possible, point being you should avoid a scope that is too high or raised as it alters your cheek position. Most high quality scopes now have more than enough elevation than needed.