Most are SKS style circle loop base, but there are also pictures show AK style dog ear front sights. Why there are 2 different style?

I'm not sure why, I doubt anyone does. Perhaps some differences/changes were seen as improvements. Some may have been cost saving measures. But the Czechoslovakians changed a number of things on the final vz. 58 over time. Including the trigger guard, FSB, sling attachment points, mag releases, bolts, locking blocks, etc.

This thread shows many, if not all of the variations:
http://www.theakforum.net/forums/66-czechoslovakian/136462-vz-58-part-variations-reference.html