I've switched from ace-jump to avy-jump just recently. One major difference is that ace-jump's overlays are displayed on top of the jump target and only one jump character is displayed at a time. With avy-mode, the overlays always show the complete jump character sequence and are displayed right of the jump target.

At least my personal preference (which is not ace-jump biased too much; I started using that only two weeks ago before switching to avy-jump yesterday) is that I like the ace-jump behavior better because the buffer contents won't switch to the right. That is, if you have 5 matches on a line, each requiring a 2-char key sequence to jump to, then the last match has switched 8 chars to the right. With ace-jump, it stays where it has been before. So this "fix target with your eyes and then jump" thingy works better.