U2412M is 1920x1200 (1200p, 16:10)
U2312HM is 1920x1080 (1080p, 16:9)
Only difference is the screen size and aspect ratio.
1200p monitors cost significantly more than 1080p. An extra inch isn't worth the cost in my opinion.
edit: If your looking for monitors for Eyefinity, AOC has some monitors with very thin bezels.