They are like completely different games. But if you must compare them..
Rust is cheaper and has a DayZ theme about it. Rust also feels really broken and kind of hard to enjoy at the moment. Also the menus can be laggy and joining servers is pretty ganky at the moment. It's fun, but it needs fixed up something serious. The resource gathering is enjoyable but the long runs to places can get boring. Also really cool to build guns and use them to hunt other players for their resources.
7 Days to die is more like minecraft, graphics are kinda bad and game is still a little buggy as well. Slow development and more expensive. Can be very fun with friends but tends to lack purpose quickly if you're not trying to kill other people or something. Completely about building to survive which is fun.
Both are gonna be good games but as for the deciding factors.
Remnants: 7days
DayZ feel: Rust
Realism: Rust
Graphics: Rust