Icon pro and ultra run same frame I have the 3500 Pro bag and also a 6000ultra bag to swap back and forth. Bag difference is simple: Weight! Pro has lots of pockets and 2 big bottle holders and due to this weighs more than the ultra that is little lighter: less pockets, only 1 bottle holder etc. Kuiu has some great youtube videos you should watch, also kuiu was running a sale on the 6000ultras last time I was on their site.
Go to YouTube and search them both (if you haven't already). They have fantastic videos in all their stuff/lines. I recently bought the Icon Pro 1850 with suspension (no frame). Can't wait to try it out this year. Nicest pack I've ever had.