For $48, you get to test out the game, look at the flaws and contribute to the improvement of the game before it's released if you want to that is. You also get to play the full routes of all the bishounen's. You get a sense of self-satisfaction doing good deeds for humanity ya know. It's like getting a sneak peek at the goodies but you don't get to devour all the goodies at the same time. $28 bucks is the full version but you'll have to be cool, calm and collected, really patient because it might take time to be released after all the flaws have been perfected in the game! As a buyer, who is independent and not at all affiliated with any game dev team, I'd recommend you to get what I've got, (the $48 one) it's worth it tbh and you won't regret it. In the end the choice is yours but take heed of my opine. ( ´ ▽ ` )b
When the game was still mostly in beta, it let you play the routes as they were being released and test them out. Currently, there is no "advantage" to it, necessarily, except supporting the creator, since they're an indie developer and are the ones doing the brunt of the work. The only thing I could maybe think of is, they might release voices in batches(since it's being first released without the voices), but we'll have to see honestly.
I had personally purchased it when they had the $51.99 tier, because it came with an artbook and I wanted to give them an amount I thought was fair for all of their hard work. It's up to you which tier you want to pay for, there's no shame in choosing the cheapest one, if you want, but if you wanted to give them more beyond that, that's fine too.