I can't find any information differentiating these two guitars, other than the F310 is wildly popular.
Yamaha.com shows no difference.
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Aside from the F-310 being discontinued in the US (as Matthew says), I believe the F-325 has a tortoiseshell pickguard, that being the only difference. My experience is that they sound nearly exactly alike; same woods (spruce, meranti, nato), same design.