They are MOSTLY the same, having many if not all of the same features. One is an enthusiast/gaming oriented board with a few aesthetic features like RGB etc. that are likely lacking on the other board, as well as some eyecandy as far as things like fancy I/O shield covers and "armor" covering the PCIe slot area.
If you are building for looks, the Rampage. If you just want the features, the Sage is probably just as good.
Personally, X299 is a bad choice anyhow unless you absolutely need support for an ungodly 128GB of memory.
Coffee lake and Z370, minus the doubled memory capacity, will fairly kick the crap out of X299 in most tasks and is likely less expensive to boot.