I do not own the 5900 but to do have the r3 3300. I have owned 3 Exos before. By far my favorite bags!!! Great features, pockets, etc! Side pockets are incredible. Problem is I like my X-Curve frame to much to change. I went with the r3 solo over the standard Solo. It has some PALS to add versatility to it. I mounted a large Kifaru pouch to the front of mine for added organization. My dream pack would be an X-CURVE frame with an Exo packbag.
I have no comparison of the exo but i can tell you that i highly enjoy my 5900. The design is great, its got a decent amount of room to stay a week, plenty or options for additional pocket storage if you want, and ive got multiple comments and how it compresses down so nicely when I'm day hunting.