How is it different from Summer memories ?
It's probably a stupid question, my bad, but it looks like Summer Memories with a winter theme and some snow.
Are there major differences ? in positions, girls or overall quality ? Is it worth it?
This has been asked before many times, it's basically a sequel but you can realistically see it as Summer Memories +++. You get the old scenes + new ones with old characters, many of the old supporting characters that only had their one and done scenes have quite a few more now and actually progress through them into more intense activities, and obviously different events around town like the new cooking minigame and so on.
It does still feel more like an expansion than a sequel, but it's still worth playing if you liked the first game as there's a good amount of new content on top of the old stuff and the three main girls start sexually open with Boku from the start, so their progression is instead moving into a more intimately sexual relationship i.e. you basically replace your aunt's husband and can even choose for him to watch by accident and cuck him if that's your thing.
As for the Kagura release, it adds an 'official' english translation and uncensored images, but it's a bit buggy atm.