The two versions are for different CPUs. Unless you have a Pentium II/AMD k6, or newer CPU, it is likely that the i686 version won't run on your system. On the other hand, the i386 version will run on all intel and AMD cpus.
Basically, the architectures are:
i386 - intel 80386
i486 - intel 80486 and Cyrix clones
i586 - Pentium and AMD K5
i686 - Pentium II/III and AMD K6
???? - Pentium IIII and AMD K7
The lower number rpms run on all CPUs, but the higher number rpms may not run on lower number CPUs, depending on the program.