The story of Winter is after Summer, some events won't simply make sense. For example, you build the relationships and "corruption" of the heroines in summer, at winter the relationship is already done. Winter is more to consolidate those relationships along showing the uncle the loser he is.
Personally the events in Summer are more fun (althought that's because I like the corruption theme I guess), but Winter Memories is an way more polished game, even many of the mini games are more fun to play. The menu is also smoother and gives you freedom for actions and even for save function, there are better seen when playing than explaining here.
