320 HS is a recent replacement for 310 HS.
Greater mp rating in 320 is more of marketing ploy than actual technical advantage. When it comes to small sensors (both 310 and 320 have same size sensors), beyond about 10-12mp, more is decidely not better. If you look at Canon's high end P&S such as S100 and G12, there is a reason why they have less mp count and bigger sensors (that allows for bigger individual pixel, which allows for better low light capability).
More mp only really matters when you are talking about bigger sensors found in full frame DSLR cameras where ability to crop is a big deal, along with technical details regarding pixel size, pixel density and etc.
While 320 HS is newer, 310 HS is a better all around camera. And currently cheaper too.
