Solid top guitars are more desirable and generally sound better. These instruments age as they are played, giving the player better tone and larger sound over the time.
Here is a quick tip to tell if the guitar you are looking at is a solid top. (Note: This will not work on painted tops)
Look at the grain of the wood on the top of the guitar. Pick a line within the grain and follow it to the sound hole. If the top is sold, the grain will flow over the edge into the sound hole. If is not solid it will stop.
Check back often for more tips and tricks!