well i'm not 100% on this but i'd say first one:
disadvantage - must make a boolean check every for loop cycle
advantage - easy to skip invisible scanlines

Well, the boolean check for every scanline could be gotten rid of by selecting a compiler option that allows for loop unrolling... By doing this, the overall speed of the program would increase, but the size of the code would also increase significantly.

hydesprojects.cjb.net