AS3 gets the nod for the graphical realism of the field and stadium. Actua's players are smooth in motion with virtually less lag in movement that the higher quality FIFA 99 players have shown. Your answer to this might lead you to which game to buy. The opening cut scene is for nostalgic soccer fans, and I was impressed with the quality and feeling Gremlin put in those scenes. A few field sounds, such as shots off the post, and a ball bouncing off the ground could use some tweaking for more realism, but overall they're not noticeable.
Background music is very good as well with a very catchy song.
It depends on what you want to sacrifice. AS3 has the smoother and more precise gameplay than FIFA 99, which still has not cleared up the lag in player movements, but FIFA 99 has the better player images. AS3 also has a more realistic field and amenities in the match that bring the game closest to the real thing than any other game. The default keyboard controls in AS3 are not ergonomically designed for keyboard players, the primary movements are done using the keypad but are customizable. Other features are overall comparable with some positives and negatives with each feature between the two games.