I recommend to try it out by yourself or go watch some gameplay videos. Words can not really describe the difference because it got a lot to do with the mechanics which is kinda a "feeling". Just like not everyone is enjoying the same car... in the same term not everyone is enjoying the same mechanics. However, this "feeling" can only be fully experienced by yourself. Currently the game is on a huge 85% discount for just a few coins... so it is not to expensive simply to try it out.
https://www.gog.com/en/game/rebel_galaxy_outlaw I do not go into details here because someone could write a book about and you may still not get it. What matters more is the general "feeling", so in this case Rebel Galaxy Outlaw is more like Wing Commander: "Privateer", Star Wars or even "Colony Wars: Red Sun", pretty straight, in general a simplified but action loaded combat with a cartoon or anime-like style.
Everspace 2 is in some way more mature, more complicated and... while it does not take itself as serious as a "space sim" it got a lot of interesting and complicated gameplay mechanics. So there is way more "RPG elements" inside, especially custom builds with a lot of things to chose from and the ingame graphic is way more realistic. It could rather be compared to a "Freelancer" or even Freespace 2 to some extend... but ultimately it got mechanics from many different space games in a pretty satisfying way. It surely is one of the best space games ever made and in my mind unable to be avoided.
Some people say "Everspace 2" is "Skyrim in Space", which got some truth in it because there are lot of areas to explore, especially dungeon-like designs, and a lot of things to discover or solve. Ultimately it is hard to describe but people who love RPG elements AND space games may enjoy Everspace 2 the most. Rebel Galaxy on the other hand is more direct, simply a pretty straight forward "action space game", more focus on action, less on RPG or exploration.
Rebel Galaxy in general is more of a "like it or hate it"... so it can be a hit or miss, but Everspace 2 is in general enjoyable by as good as any "space game lovers".