Well, you're talking two different classes of weapons. The FAL series was a battle rifle firing a full-power cartridge (most produced in 7.62 NATO) whilst the AK47 is actually a true assault rifle in the true meaning of the term. It fires an intermediate cartridge (7.62x39).
The FAL is bigger in every respect; weight, physical size, ammunition fired, longer useful range etc. The AK is lighter, smaller, easier to carry, can carry more ammo for the same given weight etc.
Both are very popluar weapons; the FAL was the main battle rifle for over 90 countries and it has been estimated that over 70 million AKs (including 47, 74, SKSU and other variants) have been produced by many countries.
For more information on the FN FAL rifles look for The FAL Rifle by Blake Stevens from Collector Grade Publications. It's not cheap but well worth the price. For information on the AK series see Kalashnikov; Machine Pistols, Assault Rifles and Machine-guns, 1945 to the present by John Walter from Greenhill Military Manuals.