From the link below it looks like the only difference is the 100MHz higher clock frequency so the a6-3420m should be slightly faster. After looking at the baseline files available there are only a few a6-3420m's in the database but tons of the a6-3400m. This may paint a false picture. Its possible because of the small number of results for the a6-3420m all tested laptops were tested in a low power mode so the APU was not running at full speed. Also I found some overclocked a6-3400m's that could be skewing the results. No guaranty but the a6-3400m will perform better after more people submit results. Also if you can wait someone from Passmark will most likely chime in. Could be a database or cpu ID problem also? If so Passmark is fast at fixing either problem if one exists. Oh if you are purchasing from a B&M store run PT on the demo model your looking at buying.
Main Box*i7 930*GA X58A-UD3R R2*3x4 gig Patriot DDR3 1600EL*EVGA GTX 460 1 gig*Intel X25-M G2 80 gig*WD Green WD20EARS 2 TB*ASUS DRW-24B3LT*Samsung SH-S223L*Corsair AX750 PSU*Rosewill Challenger case*Windows 7 Ultimate 64 bit SP1